Photomask for focus monitoring, method of focus monitoring, unit for focus monitoring and manufacturing method for a unit

ABSTRACT

A photomask for focus monitoring of the present invention is provided with a substrate that allows the exposure light to pass through and a unit mask structure for focus monitoring. Unit mask structure for focus monitoring has two patterns, and that are formed on the surface of substrate and a light blocking film that has a rear surface pattern that is formed on the rear surface of substrate for substantially differentiating the incident directions of the exposure light that enters two patterns, and for position measurement. When the dimension of rear surface pattern is L and the wavelength of the exposure light is λ, L/λ is 10, or greater.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a photomask for focusmonitoring, a method of focus monitoring, a unit for focus monitoringand a manufacturing method of the unit.

[0003] 2. Description of the Background Art

[0004] Increases in the integration and the miniaturization insemiconductor integrated circuits have been remarkable in recent years.Together with that, the miniaturization of the circuit pattern formed ona semiconductor substrate (hereinafter referred to simply as a wafer)has greatly progressed.

[0005] In particular, photolithographic technology is widely recognizedas a basic technology in the pattern formation. Accordingly, a varietyof developments and improvements have been carried out up to the presenttime. However, the miniaturization of patterns shows no signs of slowingdown and demand for increase in resolution of the patterns is on theincrease.

[0006] Such a photolithographic technology is a technology fortranscribing patterns from a photomask (original image) to a photoresistapplied on a wafer so that an etched film in the lower layer ispatterned by using this transcribed photoresist.

[0007] At this time of photoresist transcription, a developmenttreatment is carried out on the photoresist and a photoresist whereinthe portion hit by light through this development treatment is removedis called a positive type while a photoresist wherein the portion hit bylight is not removed is called a negative type photoresist.

[0008] In general, the resolution limit R (nm) in photolithographictechnology using a downscaling exposure method is represented as:

R=k ₁·λ/(NA)

[0009] Here, λ is the wavelength (nm) of the utilized light, NA is thenumerical aperture in the projection optical lens system and k₁ is aconstant depending on the image formation condition and the resistprocess.

[0010] As is seen from the above equation, there is a method of makingthe values of k₁ and λ smaller and of making the value of NA larger inorder to achieve an increase in the resolution limit R, that is to say,to gain microscopic patterns. That is to say, in addition to making theconstant, which depends on the resist process, smaller, a shortening ofthe wavelength and an increase of NA may be implemented.

[0011] From among these, a shortening of the wavelength of the lightsource is technically difficult and, therefore, it becomes necessary toincrease the NA for the same wavelength. When an increase in NA isimplemented, however, the focal depth δ(δ=k₂·λ/(NA)²) of light becomesshallow and, therefore, there are problems such that deterioration inform and in dimension precision of formed patterns is caused.

[0012] In order to expose a photoresist according to the patterns of aphotomask with a high resolution using such photolithographictechnology, it is necessary to carry out the exposure under thecondition wherein the photoresist accords with the optimal imageformation surface (optimal focus surface) of the projection opticalsystem within the range of the focal depth. Therefore, it is necessaryto precisely find the distance from the surface of the exposed substrateto the projection optical system. The process of finding this distanceis called focus monitoring.

[0013] Concerning conventional focus monitoring, there is, for example,the method of phase shift focus monitoring developed by Brunner of IBMCorporation and sold by Benchmark Technology Corporation of the UnitedStates and the phase shift focus monitoring mask that is used in thismethod.

[0014]FIG. 56 is a view for describing the operational principle of themethod of phase shift focus monitoring. In reference to FIG. 56, a phaseshift focus monitoring mask 105 is used in this method of phase shiftfocus monitoring. This phase shift focus monitoring mask 105 has atransparent substrate 105 a, a light blocking film 105 b having apredetermined pattern and a phase shifter 105 c that is formed on thispredetermined pattern.

[0015] Concretely, this phase shift focus monitoring mask 105 has apattern wherein a thin light blocking pattern 105 b is arranged betweensufficiently thick transmission portions 105 d and 105 e , as shown inFIG. 57. Here, a phase shifter 105 c is not placed in transmissionportion 105 d while a phase shifter 105 c is placed on transmissionportion 105 e.

[0016] In reference to FIG. 56, in this method of phase shift focusmonitoring, first, phase shift focus monitoring mask 105 is irradiatedwith light. At this time, since phase shifter 105 c is formed so as toshift the phase of the transmission light by approximately 90°, in thecase that the light that has passed through transmission portion 105 eprecedes the light that has passed through the transmission portion 105d by the optical path difference of 1/4 λ, 5/4 λ . . . , or in the casethat the light that has passed through transmission portion 105 esucceeds the light that has passed through the transmission portion 105d by the optical path difference of 3/4 λ, 7/4 λ, the light acts in amutually reinforcing manner. Thereby, the light after passing throughphase shift focus monitoring mask 105 has an asymmetric intensitydistribution with respect to the z axis (optical axis). This light thathas passed through phase shift focus monitoring mask 105 is condensed bymeans of projection lens 119 a and 119 b so as to form an image on aphotoresist 121 b, which is on a semiconductor substrate 121 a.

[0017] According to this method of phase shift focus monitoring, animage is formed on photoresist 121 b under the condition wherein theintensity distribution of the diffracted light is asymmetric withrespect to the z axis (optical axis: the longitudinal direction in thefigure). Therefore, an image of the pattern shifts in the direction (x-ydirection: lateral direction in the figure) perpendicular to the z axis(optical axis) on wafer 121 due to the shift of wafer 121 in the zdirection. By measuring this amount of shift of the image of the patternin the x-y direction, the measurement of the position in the zdirection, that is to say the measurement of the focus, becomespossible.

[0018] In addition to the above described method of phase shift focusmonitoring there is a method disclosed in, for example, Japanese PatentLaying-Open No. 6-120116(1994) that is a method of focus monitoring. Inthis method, a first predetermined pattern in the photomask surface isfirst irradiated with an exposure light of which the main light beam hasthe first angle of inclination and, thereby, the first image of thefirst predetermined pattern is exposed on a substrate of photosensitivematerial. After that, a second predetermined pattern that is differentfrom the above first predetermined pattern is irradiated with anexposure light of which the main light beam has a second angle ofinclination that differs from the first angle of inclination and,thereby, the second image of the second predetermined pattern is exposedon the substrate of the photosensitive material. By measuring thedistance between the exposed first and second images, the distance fromthe position of the substrate of the photosensitive material to theoptimal image formation surface can be found from the relationshipbetween this distance and the amount of defocus.

[0019] In this method, a predetermined pattern on the photomask surfaceis irradiated according to the first angle of inclination or accordingto the second angle of inclination and, therefore, a photomask 205having the structure as shown in FIG. 58 is used.

[0020] In reference to FIG. 58, this photomask 205 has a transparentsubstrate 205 a, marks for position measurement 205 b ₁, and 205 b ₂formed on the surface of this transparent substrate 205 a and adiffraction grid pattern 205 c formed on the rear surface of transparentsubstrate 205 a. That is to say, an exposure light that has struckphotomask 205 is diffracted by diffraction grid pattern 205 c so thatmark for position measurement 205 b ₁ is irradiated according to thefirst angle of inclination and mark for position measurement 205 b ₂ isirradiated according to the second angle of inclination.

[0021] In the above described phase shift focusing monitor, however, itis necessary to use a phase shift mask of a specific structure asphotomask 105. There is a problem point that the photomask becomesexpensive because a photomask of such a specific structure is necessary.

[0022] In addition, in a conventional method of phase shift focusmonitoring, it is necessary to use illumination which is isotropic(pupil plane is circular) and of which the angle spread is small, thatis to say that has a small σ value, in order to gain a high detectionsensitivity in the z direction (ratio of the of shift amount in the x-ydirection to the shift amount in the z direction). This is described inT. A. Brunner et al., “Simulations and experiments with the phase shiftfocus monitor,” SPIE Vol. 2726, pp. 236-243. In particular, FIG. 4 ofthe above reference shows that when the σ value is 0.3, the shift amountin the x-y direction of the pattern (focus monitor overlay error)becomes of the maximum and the detection sensitivity in the z directionbecomes high.

[0023] It is necessary to reduce the diameter of aperture 14 d ofillumination aperture unit 14, such as the illumination diaphram asshown in, for example, FIG. 59.

[0024] However, when the device pattern is formed by using illuminationof which the σ value is small, such as approximately 0.3, the coherenceof the light is too intense and transformation of the secondary patterntranscribed to the photoresist becomes significant. In order to preventsuch transformation of the secondary pattern, it is necessary to makethe σ value be, for example, 0.6 or higher, by making the diameter ofthe aperture of the illumination aperture unit 14 used at the timedevice pattern formation greater than the diameter of the aperture ofthe illumination aperture unit 14 used at the time of focus monitoring.Therefore, illumination aperture unit 14 must be replaced at the timebetween focus monitoring and device pattern formation and, therefore,there is a problem that labor and maintenance become necessary for thereplacement.

[0025] In addition, since at the time of replacement the lenses becomeclouded in the case that an oxygen mixture remains in the illuminationoptical system, it is necessary to carry out oxygen purging byintroducing nitrogen for a long period of time after the replacementand, therefore, there is a problem such that the operation becomescomplicated.

[0026] In addition, in a method disclosed in the Japanese PatentLaying-Open No. 6-120116(1994), it is necessary to form a diffractiongrid pattern 205 c on the rear surface of a photomask 205 as shown inFIG. 58. It is necessary for this diffraction grid pattern 205 c to be amicroscopic pattern so as to allow light to diffract. There is a problempoint that the process dimensions become small because of the necessityfor forming such a microscopic pattern and the fabrication of thephotomask becomes difficult.

[0027] In addition, it is necessary to illuminate only the portion ofthe rear surface of photomask 205 where diffraction grid pattern 205 cexists with an exposure light and, therefore, there is also a problemthat the illumination range must be concentrated in one small portion.

SUMMARY OF THE INVENTION

[0028] A purpose of the present invention is to provide a photomask forfocus monitoring, a method of focus monitoring, a unit for focusmonitoring and a manufacturing method for the unit, wherein thefabrication of the photomask is easy and the replacement of theillumination aperture unit is unnecessary while focus monitoring ofwhich the detection sensitivity in the z direction is high becomespossible.

[0029] A photomask for focus monitoring of the present invention is aphotomask for focus monitoring that is used for focus monitoring whereina position in an optical system of the exposed surface is measured inorder to adjust the focus of the optical image on the exposed surface atthe time of pattern exposure and is provided with a substrate thatallows the exposure light to pass through and unit mask structures forfocus monitoring. A unit mask structure for focus monitoring is providedwith two patterns for position measurement and a light blocking film.The two patterns for position measurement are formed on the surface ofthe substrate in order to measure the relationship between the mutualpositions. The light blocking film is formed on the rear surface of thesubstrate and has a rear surface pattern for substantiallydifferentiating the incident directions of the exposure light thatenters the two patterns for position measurement. When the dimension ofthe rear surface pattern is L and the wavelength of the exposure lightis λ, L/λ is 10 or higher.

[0030] According to the photomask for focus monitoring of the presentinvention, the dimension L of the rear surface pattern is determined sothat L/λ becomes 10 or higher, and, therefore, the dimensions of thepattern can be sufficiently enlarged to the degree such that diffractioncan be ignored. Since the process dimensions of the rear surface patternbecome enlarged in such a manner, the fabrication of the photomask forfocus monitoring becomes easy.

[0031] In addition, by sufficiently differentiating the incidentdirections of the exposure light that enters the two patterns forposition measurement through the provision of such a rear surfacepattern, a high detection sensitivity in the z direction, of which thedegree is approximately the same as that of the method of phase shiftfocus monitoring wherein the σ value is reduced using the conventionalillumination, can be gained. In addition, it is not necessary to reducethe a value as in the phase shift focusing monitoring method and,therefore, it is not necessary to replace the illumination aperture unitat the time between focus monitoring and device pattern formation.

[0032] In the above described photomask for focus monitoring, the rearsurface pattern formed in the light blocking film, on the rear surfaceof the substrate, is preferably formed so as to block a portion theexposure light that enters, at least, either of the two patterns forposition measurement and so as to allow only the remaining exposurelight to enter in the case that the light blocking film is not formed.

[0033] Thereby, the incident directions of the exposure light thatenters the two patterns for position measurement can be made to differsubstantially by providing a light blocking film having patterns.

[0034] In the above described photomask for focus monitoring, the rearsurface pattern preferably consists of a group of patterns that areshared by the two patterns for position measurement.

[0035] Thereby, it is not necessary to provide the rear surface patternsin the same number as the number of patterns for position measurement sothat the number of rear surface patterns can be reduced.

[0036] In the above described photomask for focus monitoring, the rearsurface pattern preferably consists of a pattern that is symmetricallyarranged with respect to a point of the rear surface of the substratefacing the central point between the two patterns for positionmeasurement.

[0037] Thereby, design of the rear surface pattern becomes easy.

[0038] In the above described photomask for focus monitoring, the abovedescribed pattern symmetrically arranged-with respect to the point onthe rear surface of the substrate is preferably arranged symmetricallywith respect to the second fictitious line gained by projecting thebisector perpendicular to the first fictitious line connecting twopatterns for position measurement to the rear surface of the substrate.

[0039] Thereby, telecentricity of the exposure light that irradiates thetwo patterns for position measurement can be made to be symmetric.

[0040] In the above described photomask for focus monitoring, the rearsurface pattern is preferably an aperture pattern that is substantiallycircular.

[0041] The incident direction of the exposure light that enters the twopatterns for position measurement can be made to differ substantiallyeven when such a circular aperture pattern is used.

[0042] In the above described photomask for focus monitoring, when theaperture diameter of the circular aperture pattern that becomes the rearsurface pattern is r, the depth of the substrate is D, the numericalaperture is NA and the coherence that is the coherency index of theexposure light is σ, the value of sin(tan⁻¹(r/D)) is preferably smallerthan the INA value (=NA×σ/projection magnification) that is the amountof spread of illumination.

[0043] In the case that the value of sin(tan⁻¹(r/D)) is the INA value orhigher, a portion of the exposure light that enters the patterns forposition measurement cannot be blocked depending on the arrangementposition of the rear surface pattern and, thereby, the incidentdirections of the exposure light that enters the two patterns forposition measurement cannot be made to differ.

[0044] In the above described photomask for focus monitoring, when theaperture diameter of the circular aperture pattern that becomes the rearsurface pattern is r, the depth of the substrate is D, the numericalaperture is NA and the coherence that is the coherency index of theexposure light is α, the value of sin(tan⁻¹(r/D)) is greater than thevalue of the INA value (=NA×σ/projection magnification), which is theamount of spread of illumination, multiplied by 0.1.

[0045] In the case that the value of sin(tan⁻¹(r/e)) is the value of theINA value multiplied by 0.1 or less, the amount of the exposure lightbecomes {fraction (1/100)} of the case of the conventional transcriptionor less, and it becomes difficult to transcribe the patterns forposition measurement to the photosensitive material. As a result, thethroughput of the measurement of the focus is lowered.

[0046] In the above described photomask for focus monitoring, the rearsurface pattern is preferably a pattern that allows a light blockingfilm to remain in a substantially a circular form.

[0047] The incident directions of the exposure light that enters the twopatterns for position measurement can be made to differ substantially byusing, in the above manner, the pattern that allows a circular lightblocking film to remain.

[0048] In the above described photomask for focus monitoring, when theaperture diameter of the pattern that allows a circular light blockingfilm to remain that becomes the rear surface pattern is r, the depth ofthe substrate is D, the numerical aperture is NA and the coherence thatis the coherency index of the exposure light is σ, the value ofsin(tan⁻¹(r/D)) is preferably smaller than the INA value(=NA×σ/projection magnification) that is the amount of spread ofillumination.

[0049] In the case that the value of sin(tan⁻¹(r/D)) is the INA value orhigher, a portion of the exposure light that enters the patterns forposition measurement cannot be blocked depending on the arrangementposition of the rear surface pattern and, thereby, the incidentdirections of the exposure light that enters the two patterns forposition measurement cannot be made to differ.

[0050] In the above described photomask for focus monitoring, when theaperture diameter of the pattern that allows a circular light blockingfilm to remain which becomes the rear surface pattern is r, the depth ofthe substrate is D, the numerical aperture is NA and the coherence thatis the coherency index of the exposure light is σ, the value ofsin(tan⁻¹(r/D)) is preferably greater than the value of the INA value(=NA×σ/projection magnification), which is the amount of spread ofillumination, multiplied by 0.5.

[0051] In the case that the value of sin(tan⁻¹(r/D)) is the value of theINA value multiplied by 0.5 or less, the light blocking portion becomessmall and it becomes difficult to secure the non-telecentriccharacteristics of the exposure light. As a result, the detectionsensitivity of the pattern in focus monitoring is lowered.

[0052] Now, the meaning of non-telecentric characteristics is that, forexample, the main light beam of the exposure light enters the exposedbody with an inclination relative to the optical axis of theillumination optical system.

[0053] In the above described photomask for focus monitoring, the rearsurface pattern is preferably a rectangular aperture pattern.

[0054] By using a rectangular aperture pattern in such a manner, theincident directions of the exposure light that enters the two patternsfor position measurement can be made to differ substantially.

[0055] In the above described photomask for focus monitoring, when thethickness of the substrate is D, the numerical aperture is NA and thecoherence, which is the interference indication of the exposed light, isσ, the part within the range where the distance from the sides of therectangular aperture pattern, which is the rear surface pattern, is R orless, satisfying the equation sin(tan⁻¹(R/D))≧INA value(=NA×σ/projection magnification), is preferably the light blocking partwherein a light blocking film is formed.

[0056] Thereby, only the illumination light from the rectangularaperture pattern can be made to enter each of the two patterns forposition measurement so that the non-telecentric characteristics of theexposure light can be firmly secured.

[0057] In the above described photomask for focus monitoring, therectangular aperture pattern is preferably a square aperture pattern.

[0058] In the case that a square aperture pattern is used in such amanner, the incident directions of the exposure light that enters thetwo patterns for position measurement can also be made to differsubstantially.

[0059] In the above described photomask for focus monitoring, the twopatterns for position measurement are preferably located, respectively,in the positions on the surface of the substrate that face the centerpoints of the respective sides of the square aperture pattern facingeach other.

[0060] Thereby, the two patterns for position measurement can berespectively illuminated with illumination light of which thedistribution of the incident angle is half as in the case wherein therear surface light blocking film, which is complementary, does notexist.

[0061] In the above described photomask for focus monitoring, when thelength of a side of the square aperture pattern, which is the rearsurface pattern, is a, the thickness of the substrate is D, thenumerical aperture is NA and the coherence, which is the interferenceindication of the exposure light, is σ, the value of sin(tan⁻¹(a/D)) ispreferably greater than a value twice the INA value (=NA×σ/projectionmagnification), which is the amount of spread of the illumination.

[0062] Thereby, the entirety of the maximum incident angle components ofthe illumination on the side where the aperture exists can reach thepatterns for position measurement through the above square aperturepattern.

[0063] In the above described photomask for focus monitoring, when thelength of a side of the square aperture pattern, which is the rearsurface pattern, is a, the thickness of the substrate is D, thenumerical aperture is NA and the coherence, which is the interferenceindication of the exposure light, is σ, the value of sin(tan⁻¹(a/D)) ispreferably smaller than a value three times the INA value(=NA×σ/projection magnification), which is the amount of spread of theillumination.

[0064] In the case that the value of sin(tan⁻¹(a/D)) is three timesgreater or more, than the INA value, the square aperture pattern becomestoo large and, thereby, the unit mask structure for focus monitoringbecomes too large to arrange a plurality of unit mask structures, asdescribed above, on the same mask or to carry out focus monitoring in aplurality of points within exposure field.

[0065] In the above described photomask for focus monitoring, when thelength of a side of the rectangular aperture pattern, which is the rearsurface pattern, is a, the thickness of the substrate is D, thenumerical aperture is NA and the coherence, which is the interferenceindication of the exposure light, is σ, the value of sin(tan⁻¹(a/D)) ispreferably greater than a value 0.2 times the INA value(=NA×σ/projection magnification), which is the amount of spread of theillumination.

[0066] When the value of sin(tan⁻¹(a/D)) is 0.2 times the INA value orless, the amount of the exposure light becomes {fraction (1/100)} orless of the case of a conventional transcription and it becomesdifficult to transcribe the patterns for position measurement to thephotosensitive material. As a result of this, the throughput of themeasurement of the focus is lowered.

[0067] In the above described photomask for focus monitoring, the rearsurface pattern is preferably a rectangular pattern that allows a lightblocking film to remain.

[0068] In the case that a rectangular pattern that allows a lightblocking film to remain is used in such a manner, the incidentdirections of the exposure light that enters the two patterns forposition measurement can also be made to differ substantially.

[0069] In the above described photomask for focus monitoring, when thethickness of the substrate is D, the numerical aperture is NA and thecoherence, which is the interference indication of the exposed light, isσ, the part within the range where the distance from the sides of therectangular pattern that allows a light blocking film to remain, whichis the rear surface pattern, is R, or less, satisfying the equationsin(tan⁻¹R/D))≧INA value (=NA×σ/projection magnification), is preferablythe light blocking part wherein a light blocking film is not provided.

[0070] Thereby, an aperture that allows the patterns for positionmeasurement to be exposed with a sufficient amount of light can becreated outside of the rectangular pattern that allows a light blockingfilm to remain.

[0071] In the above described photomask for focus monitoring, therectangular pattern that allows a light blocking film to remain ispreferably a square pattern that allows a light blocking film to remain.

[0072] In the case that a square pattern that allows a light blockingfilm to remain is used in such a manner, the incident directions of theexposure light that enters the two patterns for position measurement canalso be made to differ substantially.

[0073] In the above described photomask for focus monitoring, the twopatterns for position measurement are preferably located, respectively,in the positions on the surface of the substrate that face the centerpoints of the sides, respectively facing each other, of the squarepattern that allows a light blocking film to remain.

[0074] Thereby, the two patterns for position measurement can berespectively illuminated with illumination light of which thedistribution of the incident angle is half as in the case wherein therear surface light blocking film, which is complementary, does notexist.

[0075] In the above described photomask for focus monitoring, when thelength of a side of the square pattern that allows a light blocking filmto remain, which is the rear surface pattern, is a, the thickness of thesubstrate is D, the numerical aperture is NA and the coherence, which isthe interference indication of the exposure light, is σ, the value ofsin(tan⁻¹(a/D)) is preferably greater than a value twice the INA value(=NA×σ/projection magnification), which is the amount of spread of theillumination.

[0076] Thereby, the incident angle components of the illumination on theside where the light blocking pattern exists can be completely blockedso as not to reach the patterns for position measurement.

[0077] In the above described photomask for focus monitoring, when thelength of a side of the square pattern that allows a light blocking filmto remain, which is the rear surface pattern, is a, the thickness of thesubstrate is D, the numerical aperture is NA and the coherence, which isthe interference indication of the exposure light, is σ, the value ofsin(tan⁻¹(a/D)) is preferably smaller than a value three times the INAvalue (=NA×σ/projection magnification), which is the amount of spread ofthe illumination.

[0078] In the case that the value of sin(tan⁻¹(a/D)) is three timesgreater or more than the INA value, the square aperture pattern becomestoo large and, thereby, the unit mask structure for focus monitoringbecomes too large to arrange a plurality of unit mask structures, asdescribed above, on the same mask or to carry out focus monitoring in aplurality of points within exposure field.

[0079] In the above described photomask for focus monitoring, when thelength of a side of the rectangular pattern that allows a light blockingfilm to remain, which is the rear surface pattern, is a, the thicknessof the substrate is D, the numerical aperture is NA and the coherence,which is the interference indication of the exposure light, is σ, thevalue of sin(tan⁻¹(a/D)) is preferably greater than a value 0.5 timesthe INA value (=NA×σ/projection magnification), which is the amount ofspread of the illumination.

[0080] When the value of sin(tan⁻¹(a/D)) is 0.5 times the INA value orless, the light blocking part becomes small and it becomes difficult tosecure the non-telecentric characteristics of the exposure light. As aresult of this, the pattern detection sensitivity in focus monitoring islowered.

[0081] In the above described photomask for focus monitoring, one of thetwo patterns for position measurement is preferably an inner box patternof a box-in-box type while the other is an outer box pattern.

[0082] Thereby, the positional shift of the patterns for positionmeasurement in the x-y plane can be easily measured.

[0083] In the above described photomask for focus monitoring, a box edgeof, at least, either the inner box pattern or of the outer box patternis preferably formed of either the line pattern, the space pattern or aplurality of hole patterns arranged at constant intervals.

[0084] A variety of patterns can be used in such a manner for a box edgeof the box pattern.

[0085] In the above described photomask for focus monitoring, when aconstant depending on the resist process and on the image formationconditions is k₁, the wavelength of the exposure light is λ and thenumerical aperture is NA, the size S of the pattern in the box edge of,at least, either the inner box pattern or the outer box patternpreferably satisfies S=k₁×λ/NA(0.3<k_(i)<0.6).

[0086] Thereby, measurement of the focus corresponding to the actualdevice becomes possible.

[0087] In the above described photomask for focus monitoring, when thethickness of the substrate is D, the numerical aperture is NA and thecoherence, which is the interference indication of the exposure light,is σ, the distance N between the centers of the two patterns forposition measurement is preferably greater than 0.5 times and smallerthan 4 times the product (=INA value×D) of the INA value, which is theamount of spread of the illumination (=NA×σ/projection magnification),and D.

[0088] Thereby, the patterns for position measurement can beappropriately irradiated. In the case that the distance M between thetwo patterns for position measurement is 0.5 times the value of the INAvalue×D or less, neither of the two patterns for position measurementcan be diagonally irradiated. In addition, making the distance M betweenthe two patterns for position measurement four times the value of theINA value×D or. greater does not yield substantial benefits andunnecessarily increases the dimensions of the unit mask structures forfocus monitoring such that a large number of unit mask structures forfocus monitoring cannot be arranged on the mask.

[0089] The above described photomask for focus monitoring is preferablyfurther provided with a mask structure for correcting the wafer positionshift amount that has two additional patterns for position measurement,formed on the surface of the substrate, for the measurement of themutual positional relationship and a pattern formed on a light blockingfilm formed on the rear surface of the substrate for makingsubstantially equal the incident directions of the exposure light thatenters the above two additional patterns for position measurement.

[0090] It becomes possible to measure the shift amount of the amount ofmovement at the time of the carrying out of the second exposure aftermoving the exposed body, subsequent to the first exposure, by providingat least one mask structure for correcting the wafer position shiftamount.

[0091] In the above described photomask for focus monitoring, maskstructures for correcting the wafer position shift amount is preferablyformed in two or more places.

[0092] It also becomes possible to measure the shift amount in therotational direction of the exposed body by providing the maskstructures for correcting the wafer position shift amount in two or moreplaces.

[0093] In the above described photomask for focus monitoring, themaximum value of the arrangement distance between two arbitrarystructures from among mask structures for correcting the wafer positionshift amount that are formed in two or more places is preferably greaterthan ½ of the dimension in the longitudinal direction of the exposureregion for one shot.

[0094] Thereby, in the case that the exposure region for one shot isexposed while being shifted in the rotational direction, the detectionsensitivity of the shift amount in this rotational direction can beincreased.

[0095] In the above described photomask for focus monitoring, aplurality of unit mask structures for focus monitoring are preferablyformed on the substrate so that the pitch between the two neighboringunit mask structures for focus monitoring is no less than 8 mm and nomore than 20 mm.

[0096] Thereby, at the required spatial distance it becomes possible tomeasure the distance between the optical system and the surface of thesubstrate.

[0097] The method of focus monitoring according to the present inventionis a method of focus monitoring used in focus monitoring for measuringthe positions of the exposed surface in the optical system in order toadjust the focus of the optical image vis-à-vis the exposed surface atthe time of the exposure of the pattern and focus monitoring is carriedout by utilizing the characteristics that the image formed on thesurface of photosensitive material of the photomask pattern is moved inthe direction perpendicular to the optical axis when it moves in theoptical axis direction on the surface of the photosensitive material byirradiating the photomask for focus monitoring with the exposure light.The photomask for focus monitoring is provided with a substrate for theexposure light to pass through and the unit mask structures for focusmonitoring. A unit mask structure for focus monitoring has two patternsfor position measurement and a light blocking film. The two patterns forposition measurement are used to measure the mutual positionalrelationship formed on the surface of the substrate. The light blockingfilm is formed on the rear surface of the substrate and has a rearsurface pattern that makes the incident directions of the exposure lightthat enters the two patterns for position measurement substantiallydiffer. When the dimension of the rear surface pattern is L and thewavelength of the exposure light is λ, L/λ becomes 10 or greater.

[0098] According to the method of focus monitoring of the presentinvention, the dimension L of the rear surface pattern is determined sothat L/λ becomes 10 or greater, and, thereby, the dimension of thepattern can be enlarged to a degree such that the diffraction can beignored. Due to the enlargement of the process dimension of rear surfacepattern in the above manner, the fabrication of the photomask for focusmonitoring is made easy.

[0099] In addition, the incident directions of the exposure light thatenters the two patterns for position measurement are made to differsubstantially by providing such a rear surface pattern and, thereby, ahigh detection sensitivity in the z direction, to the same degree as inthe case wherein the σ value is reduced in the conventionalillumination, can be gained. In addition, since it is not necessary toreduce the σ value, it is unnecessary to replace the illuminationaperture unit at the time between focus monitoring and device patternformation.

[0100] The above described method of focus monitoring is preferablyprovided with the step of applying a photoresist to the substrate as aphotosensitive material, the step of exposing the applied photoresist toan image of the two patterns for position measurement of the photomaskfor focus monitoring, the step of forming a resist pattern by developinga patterning the exposed photoresist and the step of focus monitoringbased on the mutual distance between the respective image patterns ofthe two patterns for position measurement that are transcribed to theresist pattern.

[0101] Focus monitoring can be carried out by measuring the mutualdistance between the respective image patterns of the two patterns forposition measurement in the above manner.

[0102] In the above described method of focus monitoring, the method ofexposing the applied photoresist to an image of the two patterns forposition measurement of the photomask for focus monitoring is preferablyprovided with the first exposure step of exposing a photoresist to animage of the two patterns for position measurement of the photomask forfocus monitoring, the step of moving the substrate on which thephotoresist is applied in the direction perpendicular to the directionof the optical axis and the second exposure step of exposing thephotoresist to an image of the two patterns for position measurement ofthe photomask for focus monitoring. Either of the images of the twopatterns for position measurement to which the photoresist is exposed inthe second exposure step overlaps either of the images of the twopatterns for position measurement to which the photoresist is exposed inthe first exposure step.

[0103] In the case that the focus becomes misadjusted, either of theimages of the two patterns for position measurement that are exposed inthe first exposure step and either of the images of the two patterns forposition measurement that are exposed in the second exposure step arearranged in the x-y plane so as to shift in mutually oppositedirections. Therefore, focus monitoring can be carried out by measuringthis shift amount in the x-y plane.

[0104] In the above described method of focus monitoring, each of thetwo patterns for position measurement is preferably either the inner boxpattern or the outer box pattern of the box-in-box type mark.

[0105] The shift amount of each of the image patterns in the x-y planecan be easily measured by using the box-in-box type mark in such amanner.

[0106] In the above method of focus monitoring, the step of measuringthe mutual distance between the respective image patterns of the twopatterns for position measurement that are transcribed to the resistpattern is preferably carried out by using an overlap inspectionapparatus for inspecting the positional shift of the overlap byprocessing the images of the two image patterns that have been read in.

[0107] Thereby, the positional shift can be measured with highprecision.

[0108] In the above described method of focus monitoring, the step ofmeasuring the mutual distance between the respective image patterns ofthe two patterns for position measurement transcribed to the resistpattern is preferably carried out by observing the positions of the twoimage patterns by using a scanning-type electron microscope.

[0109] Thereby, the positional shift can be easily measured.

[0110] In the above described method of focus monitoring, the twopatterns for position measurement are preferably formed so that, atleast, either of the image patterns of the two patterns for positionmeasurement is readable by a pattern position detection mechanismintegrally attached to the exposure unit.

[0111] Thereby, measurement becomes possible by means of a unit having asimple configuration.

[0112] In the above described method of focus monitoring, focusmonitoring is preferably carried out by measuring the electricalresistance of the conductive layer, which is the lower layer of theresist pattern etched by using the resist pattern gained by developingthe photoresist exposed through the first and second exposure steps as amask.

[0113] Focus monitoring can be carried out by measuring the electricalcharacteristics in such a manner.

[0114] In the above described method of focus monitoring, error in theamount of movement of the substrate in the step of moving the substratein the direction perpendicular to the direction of the optical axis ispreferably measured. The error in the amount of movement is subtractedfrom the amount of positional shift between one of the images of the twopatterns for position measurement that is exposed to the photoresist inthe second exposure step and the other image of the two patterns forposition measurement that is exposed to the photoresist in the firstexposure step.

[0115] Thereby, the error in the amount of shift can be subtracted sothat focus monitoring can be carried out with increased precision.

[0116] In the above described method of focus monitoring, the photomaskfor focus monitoring is preferably further provided with a maskstructure for correcting the wafer position shift amount having twoadditional patterns for position measurement for measuring therelationship between the mutual positions formed on the surface of thesubstrate and a pattern formed on the light blocking film formed on therear surface of the substrate for allowing the incident directions ofthe exposure light that enters the above additional two patterns forposition measurement to become substantially equal. The above describedtwo additional patterns for position measurement are exposed to thephotoresist through the first and second exposure steps. An error in theamount of movement is measured from the amount of positional shift ofthe pattern formed by one of the above described two additional patternsfor position measurement being exposed to the photoresist through thefirst exposure step relative to the pattern formed by the other of theabove described two additional patterns for positional measurement beingexposed to the photoresist through the second exposure step.

[0117] Thereby, the error in the amount of movement can be subtracted sothat focus monitoring can be carried out with increased precision.

[0118] In the above described method of focus monitoring, patternsgained through exposing the photoresist to a plurality of shots bychanging the amount of focus offset of the exposure unit are preferablyused to find the relationship between the amount of positional shift ofthe pattern and the amount of focus shift in advance so that the amountof focus shift is found from the amount of positional shift of themeasured pattern based on the above relationship.

[0119] The amount of focus shift can be corrected in such a manner.

[0120] A unit for focus monitoring of the present invention is a unitfor focus monitoring used in focus monitoring for measuring the positionof the exposed surface in the optical system in order to adjust thefocus of the optical image vis-à-vis the exposed surface at the time ofpattern exposure and is provided with a photomask for focus monitoring,an illumination optical system and a projection optical system. Apattern is formed on the photomask for focus monitoring. Theillumination optical system is used to irradiate the photomask for focusmonitoring with the exposure light. The projection optical system isused to project the image of the pattern of the photomask for focusmonitoring onto the photosensitive body. The photomask for focusmonitoring is provided with a substrate for allowing the exposure lightto pass through and a unit mask structure for focus monitoring. The unitmask structure for focus monitoring has two patterns for positionmeasurement and a light blocking film. The two patterns for positionmeasurement are formed on the surface of the substrate and are used tomeasure the relationship between the mutual positions. The lightblocking film is formed on the rear surface of the substrate and has therear surface pattern for allowing the incident directions of theexposure light that enters the two patterns for position measurement todiffer substantially. When the dimension of the rear surface pattern isL and the wavelength of the exposure light is λ, L/λ becomes 10 orgreater.

[0121] According to unit for focus monitoring of the present invention,the dimension L of the rear surface pattern is determined so that L/λbecomes 10 or greater, and, therefore, a pattern of large dimensions canbe gained to the degree that diffraction can be ignored. The processdimensions of the rear surface pattern are increased in such a mannerand, thereby, the fabrication of the photomask for focus monitoringbecomes easy.

[0122] In addition, the incident directions of the exposure light thatenters the two patterns for position measurement are substantiallydifferentiated by provided such a rear surface pattern and, thereby, ahigh detection sensitivity in the z direction at approximately the samelevel as in the case where the σ value is reduced using the conventionalillumination can be gained. In addition, since it is not necessary toreduce the a value, it is unnecessary to replace the illuminationaperture unit at the time between focus monitoring and device patternformation.

[0123] A manufacturing method of a device of the present invention ischaracterized by the use of a method of focus monitoring described inany of the above described aspects.

[0124] Thereby, focus monitoring becomes possible wherein thefabrication of a photomask is easy, the replacement of the illuminationaperture unit is unnecessary and the detection sensitivity in the zdirection is high and, thereby, patterns can be formed at a low cost andwith a high precision.

[0125] In the above described manufacturing method of a device, thedevice that is formed by using the above described method of focusmonitoring is preferably a semiconductor device.

[0126] The above described manufacturing method of a device is suitablefor the manufacture of a device that is able to be manufactured by usinga semiconductor manufacturing process such as a thin film magnetic filmor a liquid crystal display element and is also suitable for themanufacture of a semiconductor device.

[0127] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0128]FIG. 1 is a schematic view for describing a method of focusmonitoring according to a first embodiment of the present invention;

[0129]FIG. 2 is an enlarged view of the rear surface aperture pattern ofthe photomask of FIG. 1 for describing the function thereof;

[0130]FIG. 3 is a schematic view showing the configuration of a unit forfocus monitoring according to the first embodiment of the presentinvention;

[0131]FIGS. 4A and 4B are a view of the rear surface and a view of thetop surface schematically showing the configuration of the photomask forfocus monitoring according to the first embodiment of the presentinvention;

[0132]FIG. 5 is a schematic cross sectional value along line V-V ofFIGS. 4A and 4B;

[0133]FIG. 6 is a view showing the configuration of an annular bandillumination diaphram that is used as the diaphram in FIG. 3;

[0134]FIG. 7 is a view showing the appearance of the aperture pattern onthe rear surface when the rear surface of the substrate is seen from thepattern 5 b ₁ for position measurement;

[0135]FIG. 8 is a view showing the appearance of the aperture pattern onthe rear surface when the rear surface of the substrate is seen from thepattern 5 b ₂ for position measurement;

[0136]FIG. 9 is a view showing the region that allows light to pass tothe pattern 5 b ₁ for position measurement when the rear surface of thesubstrate is seen from the pattern 5 b ₁ for position measurement;

[0137]FIG. 10 is a view showing the region that allows light to pass tothe pattern 5 b ₂ for position measurement when the rear surface of thesubstrate is seen from the pattern 5 b ₂ for position measurement;

[0138] FIGS. 1A1 and 11B are views for describing the steps of doubleexposure according to the order of steps;

[0139]FIGS. 12A and 12B are a cross sectional view and a plan viewshowing the appearance of the photoresist wherein a box-in-box patternis formed;

[0140]FIGS. 13A, 13B and 13C are figures for describing the steps oftwice repeating the exposure and the development according to the orderof the steps;

[0141]FIG. 14 is a cross sectional view schematically showing theconfiguration of the photomask for focus monitoring for preciselycarrying out positioning at the time of the movement of the wafer;

[0142]FIG. 15 is a view for describing positioning of the wafer by usingthe photomask shown in FIG. 14;

[0143]FIGS. 16A and 16B are a view of the rear surface and a view of thetop surface showing the configuration of a photomask for focusmonitoring for carrying out focus monitoring by measuring the electricalresistance of the pattern;

[0144]FIG. 17 is a schematic cross sectional view along line XVII-XVIIin FIGS. 16A and 16B;

[0145]FIGS. 18A, 18B and 18C are figures for describing a method formeasuring focus shift by using the electrical resistance value;

[0146]FIG. 19 is a graph showing the relationship between the mutualshift of the patterns and the height of the substrate according to thefirst embodiment of the present invention;

[0147]FIG. 20 is an enlarged cross sectional view showing a portion ofthe photomask shown in FIGS. 4A, 4B and 5;

[0148]FIGS. 21A and 21B are a view of the rear surface and a view of thetop surface schematically showing the configuration of a photomask forfocus monitoring according to a second embodiment of the presentinvention;

[0149]FIG. 22 is a schematic cross sectional view along line XXII-XXIIin FIGS. 21A and 21B;

[0150]FIG. 23 is a view for describing wherein an aperture pattern 5 d ₁on the rear surface is solely distributed within a range wherein pattern5 b ₁ for position measurement can be irradiated with the exposure lightwhen the rear surface of the substrate 5 a is seen from the pattern 5 b₁ for position measurement;

[0151]FIG. 24 is a view for describing wherein an aperture pattern 5 d ₂on the rear surface is solely distributed within a range wherein pattern5 b ₂ for position measurement can be irradiated with the exposure lightwhen the rear surface of the substrate 5 a is seen from the pattern 5 b₂ for position measurement;

[0152]FIGS. 25A and 25B are a view of the rear surface and a view of thetop surface of a photomask showing the configuration, which is theconfiguration of a photomask for focus monitoring according to a thirdembodiment of the present invention, wherein the two respective patternsfor measurement are illuminated from the two aperture patterns on therear surface;

[0153]FIG. 26 is a schematic cross sectional view along line XXVI-XXVIin FIGS. 25A and 25B;

[0154]FIG. 27 is a view for describing wherein aperture patterns 5 d ₁and 5 d ₂ on the rear surface are distributed within a range whereinpattern 5 b ₁ for position measurement can be irradiated with theexposure light when the rear surface of the substrate 5 a is seen fromthe pattern 5 b ₁ for position measurement;

[0155]FIG. 28 is a view for describing wherein aperture patterns 5 d ₁and 5 d ₂ on the rear surface are distributed within a range whereinpattern 5 b ₂ for position measurement can be irradiated with theexposure light when the rear surface of the substrate 5 a is seen fromthe pattern 5 b ₂ for position measurement;

[0156]FIGS. 29A and 29B are a view of the rear surface and a view of thetop surface schematically showing the configuration of a photomask forfocus monitoring, which is the configuration of a photomask for focusmonitoring according to a fourth embodiment of the present invention,wherein a pattern on the rear surface is a circular pattern that allowsa light blocking film remain;

[0157]FIG. 30 is a schematic cross sectional view along line XXX-XXX inFIGS. 29A and 29B;

[0158]FIG. 31 is a view for describing wherein pattern 5 d is located soas to allow a light blocking film to remain solely in a portion within aregion wherein pattern 5 b ₁ for position measurement can be irradiatedwith the exposure light when the rear surface of substrate 5 a is seenfrom pattern 5 b ₁ for position measurement;

[0159]FIG. 32 is a view for describing wherein pattern 5 d is located soas to allow a light blocking film to remain solely in a portion within aregion wherein pattern 5 b ₂ for position measurement can be irradiatedwith the exposure light when the rear surface of substrate 5 a is seenfrom pattern 5 b ₂ for position measurement;

[0160]FIGS. 33A and 33B are a view of the rear surface and a view of thetop surface schematically showing the configuration of a photomask forfocus monitoring, which is the configuration of a photomask for focusmonitoring according to a fifth embodiment of the present invention,wherein a pattern on the rear surface is a rectangular pattern thatallows a light blocking film to remain;

[0161]FIG. 34 is a schematic cross sectional view along line XXIV-XXIVin FIGS. 33A and 33B;

[0162]FIGS. 35A and 35B are a view of the rear surface and a view of thetop surface schematically showing the configuration of a photomask forfocus monitoring, which is the configuration of a photomask for focusmonitoring according to a sixth embodiment of the present invention,wherein a pattern on the rear surface is a rectangular aperture pattern;

[0163]FIGS. 36A and 36B are a view of the rear surface and a view of thetop surface schematically showing the configuration of a photomask usedin a method of focus monitoring according to a seventh embodiment of thepresent invention;

[0164]FIG. 37 is a schematic cross sectional view along lineXXXVII-XXXVII in FIGS. 36A and 36B;

[0165]FIG. 38 is a view showing the position of a square aperturepattern 5 d in the case that the rear surface of substrate 5 a is seenfrom pattern 5 b ₁ for position measurement;

[0166]FIG. 39 is a view showing the position of a square aperturepattern 5 d in the case that the rear surface of substrate 5 a is seenfrom pattern 5 b ₂ for position measurement;

[0167]FIG. 40 is a view showing that the light for illuminating pattern5 b ₁ for position measurement is only the illumination light of theright half of the zonal illumination;

[0168]FIG. 41 is a view showing that the light for illuminating pattern5 b ₂ for position measurement is only the illumination light of theleft half of the zonal illumination;

[0169]FIGS. 42A, 42B and 42C are graphs showing a change in the imageintensity due to a change in focus position in the case that one patternis illuminated by using half illumination of the right and left sides,respectively;

[0170]FIG. 43 is a view showing the appearance of an image formed by oneelement illumination from among the respective half illumination whenonly the even aberration exists;

[0171]FIG. 44 is a view showing the appearance of an image formed by oneelement illumination from among the respective half illumination whenonly the odd aberration exists;

[0172]FIG. 45 is a view showing the motion of the transcription positionof a mask pattern in a method of focus monitoring according to theseventh embodiment of the present invention in the case that only theeven aberration exists;

[0173]FIG. 46 is a view representing, as a relation of the substrateheight, the mutual positional shift of the patterns resultingrespectively from the half illumination, of the right and left sides,that is an experimental measurement amount in the case that only theeven aberration exists;

[0174]FIG. 47 is a view showing the motion of the transcription positionof a mask pattern in the method of focus monitoring according to theseventh embodiment of the present invention in the case that the oddaberration also exists, in addition to the even aberration;

[0175]FIG. 48 is a view representing, as a relation of the substrateheight, the mutual positional shift of patterns resulting respectivelyfrom half illumination of the right and left sides that are theexperimental measurement amount in the case that the odd aberration alsoexists, in addition to the even aberration;

[0176]FIGS. 49A and 49B are a view showing the appearance of irradiationwith the exposure light of the pattern in the actually transcribedpattern and a graph showing the relationship between the stage controlvalue of the substrate height and the pattern dimensions;

[0177]FIGS. 50A and 50B are a view of the rear surface and a view of thetop surface schematically showing another configuration of the photomaskused in a method of focus monitoring according to an eighth embodimentof the present invention;

[0178]FIG. 51 is a schematic cross sectional view along line LI-LI inFIGS. 50A and 50;

[0179]FIG. 52 is a view showing the pattern for position measurement asa line pattern formed of a remaining light blocking film in a squareframe form;

[0180]FIG. 53 is a view showing the pattern for position measurement asa space pattern formed of a square aperture in a frame form;

[0181]FIG. 54 is a view showing the pattern for position measurement asa pattern wherein a plurality of hole patterns are arranged in a squareform;

[0182]FIG. 55 is a view showing the appearance wherein a plurality ofunit focus monitor structures are arranged in a photomask;

[0183]FIG. 56 is a view for describing a method of phase shift focusmonitoring according to a prior art;

[0184]FIG. 57 is a view showing the configuration of a photomask used inthe method of phase shift focus monitoring according to the prior art;

[0185]FIG. 58 is a schematic cross sectional view showing theconfiguration of a photomask used in the method of focus monitoringdisclosed in Japanese Patent Laying-Open No. 6-120116(1994); and

[0186]FIG. 59 is a view showing the configuration of an illuminationdiaphram used in the method of phase shift focus monitoring according tothe prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0187] In the following, the embodiments of the present invention aredescribed in reference to the drawings.

[0188] First Embodiment

[0189] First, the principle of a method of focus monitoring of thepresent embodiment is described.

[0190] In reference to FIG. 1, in a method of focus monitoring accordingto the present embodiment, a photomask 5 for focus monitoring having apattern (hereinafter referred to as rear surface pattern) 5 d formed ona light blocking film 5 c on the rear surface of the substrate is used.This photomask 5 has a substrate 5 a that allows the exposure light topass through and a light blocking film 5 c formed on the rear surface ofsubstrate 5 a and, in addition, has a unit structure for focusmonitoring (region Q surrounded by a broken line frame) formed of twopatterns 5 b ₁ and 5 b ₂ for position measurement formed on the surfaceof substrate 5 a and light blocking film 5 c on the rear surface of thesubstrate.

[0191] This aperture pattern 5 d on the rear surface of the substrate isformed so as to block a portion of exposure light 51 with which each ofthe two patterns, 5 b ₁ and 5 b ₂, for position measurement would, inthe absence of light blocking film, be irradiated as shown in FIG. 2 andso as to allow the remaining portion of the exposure light to passthrough so that each of the two patterns are irradiated.

[0192] In reference to FIG. 1, patterns 5 b ₁ and 5 b ₂ for positionmeasurement are arranged on both sides of the optical axis (line A-A)that passes through aperture pattern 5 d on the rear surface of thesubstrate. Therefore, the inside direction of exposure light 51 thatpasses through aperture pattern 5 d on the rear surface of the substrateand enters pattern 5 b ₁ for position measurement and the incidentdirection of exposure light 51 that enters pattern 5 b ₂ for positionmeasurement are different from each other. In particular, in the casethat patterns 5 d ₁ and 5 d ₂ for position measurement are arrangedsymmetrically relative to the optical axis A-A that passes throughaperture pattern 5 d on the rear surface of the substrate, the incidentdirection of exposure light 51 that enters pattern 5 d ₁ for positionmeasurement and the incident direction of exposure light 51 that enterspattern 5 d ₂ for position measurement become symmetrical relative tothe optical axis direction A-A.

[0193] The aperture dimension L0 of this aperture pattern 5 d on therear surface of the substrate is set so as to satisfy the condition ofL0/λ≧10 when the wavelength of exposure light 51 with which thisphotomask 5 is irradiated.

[0194] In the method of focus monitoring according to the presentembodiment, the patterns are irradiated exposure light 51 from the rearsurface of this photomask 5. Thereby, each of the patterns, 5 b ₁ and 5b ₂, for position measurement is irradiated only with diagonalcomponents of exposure light 51 that enter photomask 5. Therefore,diffraction light 52, which is diffracted by respective patterns 5 b ₁and 5 b ₂ for position measurement, passes through projection lenses 19a and 19 b as well as pupil plane diaphram 25 so as to form an image ona photosensitive material (for example, photoresist) 21 b in thediagonal direction.

[0195] The image of the pattern on the surface of photosensitivematerial 21 a is formed of the incident illumination light in thediagonal direction and, thereby, when a wafer 21 is moved in the zdirection (direction of optical axis A-A) the image moves in the x-ydirection (direction perpendicular to optical axis A-A) on the surfaceof photosensitive material 21 b. Concretely, when wafer 21 is moved inthe z direction so as to approach photomask 5, the respective images ofpatterns 5 b ₁ and 5 b ₂ for position measurement move away from eachother on the surface of photosensitive material 21 b. In addition, whenwafer 21 is moved in the z direction so as to be moved far away fromphotomask 5, the respective images of patterns 5 b ₁ and 5 b ₂ forposition measurement move so as to come closer to each other on thesurface of photosensitive material 21 b.

[0196] The positional relationship of the images of patterns 5 b ₁ and 5b ₂ for position measurement that are formed on photosensitive material21 b changes relatively in the x-y plane according to the movement ofwafer 21 in the z direction. By measuring this positional relationship,it becomes possible to detect the shift of focus (shift in the zdirection) when the patterns are transcribed.

[0197] Next, a unit for focus monitoring and a photomask for focusmonitoring according to the present embodiment are concretely described.

[0198] In reference to FIG. 3, this unit for focus monitoring has asimilar configuration to a scaling down projection exposure unit(stepper) and projects the scaled down pattern on photomask 5 tophotoresist 21 b on the surface of wafer 21. This unit for focusmonitoring has an illumination optical system through which light passesstarting from a light source 11 and reaching the pattern of photomask 5as well as a projection optical system through which light passesstarting from the pattern of photomask 5 and reaching wafer 21.

[0199] The illumination optical system has a mercury lamp 11 that is thelight source, a reflecting mirror 12, a condensing lens 18, a fly-eyelens 13, an diaphram 14, condensing lenses 16 a, 16 b, 16 c, a blinddiaphram 15 and a reflecting mirror 17. In addition, the projectionoptical system has projection lenses 19 a, 19 b and pupil plane diaphram25.

[0200] In the exposure operation, as for light 11 a emitted from mercurylamp 11 only the g string (wavelength: 436 nm), for example, isreflected by reflecting mirror 12 and light 11 a becomes light of asingle wavelength. Next, light 11 a passes through condensing lens 18and enters each of fly-eye component lenses 13 a forming fly-eye lens 13and, after that, passes through diaphram 14.

[0201] Here, a light 11 b shows a light path created by one fly-eyecomponent lens 13 a while light 11 c shows a light path created byfly-eye lens 13.

[0202] Light 11 a that has passed through diaphram 14 passes throughcondensing lens 16 a, a blind diaphram 15 and a condensing lens 16 b soas to be reflected by reflecting mirror 17 at a predetermined angle.

[0203] The entirety of the surface of photomask 5, on which apredetermined pattern is formed, is uniformly irradiated with light 11 athat is reflected by reflecting mirror 17 after the light passes throughcondensing lens 16 c. After this, light 11 a is scaled down according toa predetermined scaling factor by projection lenses 19 a and 19 b so asto expose photoresist 21 b on semiconductor substrate 21 a.

[0204] In reference to FIGS. 4A, 4B and 5, light blocking film 5 bhaving two patterns 5 b ₁ and 5 b ₂ for position measurement is formedon the surface of substrate 5 a that allows the exposure light to passthrough. In addition, light blocking film 5 c having a circular aperturepattern 5 d is formed on the rear surface of substrate 5 a. A unit maskstructure for focus monitoring is formed of these two patterns 5 b ₁ and5 b ₂ for position measurement and light blocking film 5 c that hasaperture pattern 5 d on the rear surface of the substrate.

[0205] Each of the two patterns, 5 b ₁ and 5 b ₂, for positionmeasurement correspond to an inner box pattern and an outer box patternof the box-in-box type mark, respectively. Pattern 5 b ₁ for positionmeasurement that becomes the inner box pattern is an approximatelysquare aperture pattern, while pattern 5 b ₂ for position measurementthat becomes the outer box pattern is an aperture pattern in a squareframe form. Aperture pattern 5 d on the rear surface of the substrate isformed in a circle, of which the center is point c2, on the rear surfaceof the substrate that is directly opposite to center point c1 of thefictitious line connecting the above two patterns 5 b ₁ and 5 b ₂ forposition measurement..

[0206] Aperture dimension LO (=2 r) of this aperture pattern 5 d on therear surface of the substrate is set so as to satisfy the condition ofL0/λ≧10 when the wave length of the exposure light with which thisphotomask 5 is irradiated is λ.

[0207] This aperture pattern 5 d on the rear surface of the substrate isprovided so as to be shared by two patterns 5 b ₁ and 5 b ₂ for positionmeasurement. Here, “provided so as to be shared” means “provided so thatboth patterns 5 b ₁ and 5 b ₂ for position measurement can be irradiatedwith light from one aperture pattern 5 d on the rear surface of thesubstrate.”

[0208] In addition, “is directly opposite to” in the above means thatpoint c2 is a point on the rear surface of photomask 5 opposite tocenter point c1 located on the top surface of photomask 5 in thedirection of the optical axis.

[0209] Next, a concrete method of focus monitoring according to thepresent embodiment is described.

[0210] In the method of focus monitoring according to the presentembodiment, a zonal illumination diaphram as shown in FIG. 6 is, forexample, used as diaphram 14 of the unit for focus monitoring in FIG. 3.At this time, a view of aperture pattern 5 d provided on the rearsurface of photomask 5 as is seen from pattern 5 b ₁ for positionmeasurement of photomask 5 is shown in FIG. 7 while a view of aperturepattern 5 d provided on the rear surface of photomask 5 as is seen frompattern 5 b ₂ for position measurement of photomask 5 is shown in FIG.8.

[0211] As shown in FIGS. 7 and 8, aperture pattern 5 d on the rearsurface of the photomask 5 is circular and therefore, when a commondiaphram is used as diaphram 14, each of patterns, 5 b ₁ and 5 b ₂, forposition measurement is irradiated with light from the entirety ofcircular aperture pattern 5 d on the rear surface of the photomask 5. Inthe case that zonal illumination diaphram 14 as shown in FIG. 6 is used,however, each of patterns, 5 b ₁ and 5 b ₂, for position measurement isirradiated with the exposure light from the remaining region of thecircular region of aperture pattern 5 d on the rear surface of photomask5, except for a portion (hatched region in the direction towards thelower left) as shown in FIGS. 9 and 10. Then, the diffraction lightdiffracted by each of the patterns, 5 b ₁ and 5 b ₂, for positionmeasurement passes through projection lenses 19 a and 19 b as well aspupil plane diaphram 25 so as to form an image on photoresist 21 b inthe diagonal direction.

[0212] Here, a concentric double circle shown in FIGS. 7 and 8 shows theangle direction in which the illumination light would enter in theabsence of the light blocking film.

[0213] In the case that the box-in-box type mark pattern is exposed adouble shift exposure is carried out. That is to say, after the firstexposure is carried out in the above described manner, wafer 21 is moved(shifted) in the x-y plane by means of an x-y stage as shown in thebottom portion in FIG. 1 and, then, the second exposure is carried out.This second exposure is carried out in the same manner as in the firstexposure. Thereby, the box-in-box pattern can be formed by, for example,placing inner box pattern 5 b ₁ exposed in the first exposure in outerbox pattern 5 b ₂ exposed in the second exposure.

[0214] More concretely, in the first exposure, photoresist 21 b isexposed in pattern 5 b ₁ for position measurement, which is the innerbox pattern, as shown in FIG. 11A. After this, wafer 21 is moved asdescribed above. After this, in the second exposure, the region that hasbeen exposed in pattern 5 b ₁ for position measurement is exposed inpattern 5 b ₂ for position measurement that becomes the outer boxpattern as shown in FIG. 11B. Thereby, the box-in-box type patternwherein inner box pattern 22 is positioned within outer box pattern 23as shown in FIGS. 12a and 12 b is formed on photoresist 21 b.

[0215] Here, light beams for forming an optical image wherein inner boxpattern 22, in the above described box-in-box, is formed, enterphotoresist 21 b in the direction towards the lower right as shown inFIG. 1 while light beams for forming an optical image wherein outer boxpattern 23 is formed enter photoresist 21 b in the direction towards thelower left. Therefore, in the case that there is a focus shift at thetime of exposure, inner box pattern 22 and outer box pattern 23 areformed so as to shift away from each other.

[0216] In FIG. 1, in the case that wafer 21 is shifted toward thephotomask 5 side, for example, from the optimal focus position at thetime of exposure, inner box pattern 22 is formed in a position shiftedtoward the right side from the position at the time of optimal focuswhile outer box pattern 23 is formed in a position shifted to the leftside from the position at the time of optimal focus as shown in FIG.12B.

[0217] Here, in FIG. 12B, each of the positions of inner box pattern 22and outer box pattern 23 at the time of optimal focus is shown by dottedlines while each of the positions of inner box pattern 22 and outer boxpattern 23 under the condition wherein there is a focus shift is shownby solid lines.

[0218] Next, intervals x1 and x2 between inner box pattern 22 and outerbox pattern 23 that have been created in the above manner are measured.Interval x3 (=(x1+x2)/2) between inner box pattern 22 and outer boxpattern 23 at the time of optimal focus is derived from these values.The difference between the above interval x3 and interval x1 or x2 isderived and, thereby, the amount of lateral movement (amount ofpositional shift) of inner box pattern 22 and outer box pattern 23 canbe found.

[0219] By comparing this amount of lateral movement with therelationship between the amount of lateral movement that has beenmeasured in advance and the focus, it becomes possible to detect thefocus shift. Thereafter, the position of wafer 21 in the z direction isadjusted based on this focus shift so that the optical focus can begained.

[0220] Here, the above described relationship between the amount oflateral movement and the focus can be gained by finding, in advance, therelationship between the amount of positional shift and the amount offocus shift using a plurality of shot patterns to which the photoresistis exposed while changing the focus offset amount of the exposure unit.

[0221] In addition, though in FIGS. 11A and 11B, a case whereinphotoresist 21 b is developed after a double exposure is described,photoresist 21 b may be patterned by repeating the step of developmentafter an exposure twice. That is to say, photoresist 21 b may bepatterned according to the steps of first exposure→firstdevelopment→movement of wafer 21→second exposure→second development.

[0222] In reference to FIG. 13A, photoresist 21 b is, first, exposed topattern 5 b ₁ for position measurement that becomes the inner boxpattern through the first exposure. After this, the first development iscarried out. Thereby, inner box pattern 22 is formed on photoresist 21 bas shown in FIG. 13B. Wafer 21 is moved in the lateral direction in thex-y plane under the condition wherein the above inner box pattern 22 isformed. Then, as shown in FIG. 13C, the exposure of pattern 5 b ₂ forposition measurement, which becomes the outer box pattern, is carriedout as the second exposure so as to overlap the region wherein inner boxpattern 22 is formed. After this, outer box pattern 23 is formed asshown in FIGS. 12A and 12B by carrying out the second development onphotoresist 21 b so that the resist pattern is formed.

[0223] Here, in the case that the step of development after the exposureis repeated twice in the above manner, the process for focus monitoringafter development is the same as in the above described case wherein twoexposures are carried out before one development is carried out.

[0224] Next, a method for correcting an error in the positioning ofwafer 21 that occurs when wafer 21 is moved between the above describedfirst exposure and second exposure is described.

[0225] In a double exposure with shift wherein the first pattern formedthrough the first exposure is overlapped with the second pattern that isformed in a different position on the mask is formed through the secondexposure, wafer 21 is moved by the distance corresponding to thepositional relationship between the above described first pattern andsecond pattern on the mask before the second exposure is carried out. Atthis time, however, the amount of movement of the wafer must beprecisely of the above described distance or the above described firstpattern and second pattern will be formed in the condition wherein theyare shifted from each other. As described above, in the present methodwherein a focus shift is detected due to the mutual positional shift ofthe first pattern and second pattern, the shift due to this error in theamount of movement of the wafer is recognized as a focus shift so thatan error will be caused in the measurement of the actual amount of focusshift.

[0226]FIG. 14 is a cross sectional view schematically showing theconfiguration of a photomask for focus monitoring that is used tomeasure this error in the amount of movement of the wafer by usinganother mask structure that is additionally formed on the same mask andthat is used to correct the apparent focus shift due this error in theamount of movement of the wafer. In reference to FIG. 14, two pairs, ormore, of the mask structures N for correcting the amount of shift in thewafer position are formed in the above photomask 5 in addition to theunit mask structure Q for focus monitoring that has the above describedpatterns 5 b ₁ and 5 b ₂ for position measurement and a light blockingfilm 5 c having aperture pattern 5 d on the rear surface of thephotomask.

[0227] This mask structure N for correcting the amount of shift in waferposition has a pair of patterns, 5 e ₁ and 5 e ₂, for positionmeasurement that is formed on the top surface of substrate 5 a. In thismask structure N for correcting the amount of shift in wafer position, alarge aperture pattern is positioned on the rear surface side ofsubstrate 5 a, wherein a light blocking film is not formed, in order toallow the respective incident directions of the exposure light thatenters this pair of patterns 5 e ₁ and 5 e ₂ for measurement to besubstantially equal.

[0228] The correction of the apparent focus shift due to the error inthe amount of movement of the wafer by using photomask 5 as shown in theabove described FIG. 14 is carried out as shown in FIG. 15. In referenceto FIG. 15, the first exposure is carried out by using photomask 5 forfocus monitoring having a pair of patterns 5 e ₁ and 5 e ₂ for positionmeasurement. At this time, though two patterns 5 b ₁ and 5 b ₂ forposition measurement are irradiated with exposure light in the diagonaldirection, the pair of patterns 5 e ₁ and 5 e ₂ for position measurementbecomes, as a whole, the same as in the case wherein only theillumination components in the direction of the optical axis exist asshown in the figure due to the symmetrical distribution of illuminationrelative to the optical axis.

[0229] After this first exposure, photoresist 21 b is developed, or isnot developed, before wafer 21 is moved. This movement is carried out sothat the exposure region of pattern 5 b ₂ for position measurementoverlaps, due to the second exposure, the region that is exposed topattern 5 b ₁ for position measurement due to the first exposure and sothat the exposure region of pattern 5 e ₂ for position measurementoverlaps, due to the second exposure, the region that is exposed topattern 5 e ₁ for position measurement due to the first exposure.

[0230] After this movement, the second exposure is carried out in thesame manner as the first exposure. After this, photoresist 21 b isdeveloped. Then, the amount of positional shift L2 is measured betweenthe image pattern of pattern 5 e ₁ for position measurement formedaccording to the first exposure of the developed resist pattern 21 b andthe image pattern of pattern 5 e ₂ for position measurement formedaccording to the second exposure. This amount of positional shift L2shows an error in the movement of wafer 21 because the image formed ofmask patterns 5 e ₁ and 5 e ₂ does not shift in the lateral directiondue to the movement of the focus. It becomes possible to preciselymeasure the amount of lateral movement due to defocusing by subtractingthe error in the movement, detected as described above, from the abovefound amount of lateral movement (amount of positional shift) of twopatterns 5 b ₁ and 5 b ₂ for position measurement.

[0231] Here, though in the above the case wherein the box-in-box typepattern is used is described, focus monitoring can be carried outaccording to the following method without using the box-in-box typepattern.

[0232] In reference to FIG. 1, the image formed on photoresist 21 b ofpatterns 5 b ₁ and 5 b ₂ for position measurement is moved in thelateral direction (x-y direction) in the figure due to the movement ofthe wafer 21 in the z direction. Therefore, the interval L1 of thepatterns of the image formed of patterns 5 b ₁ and 5 b ₂ for positionmeasurement is measured so as to be compared with the relationshipbetween the position of wafer 21 in the z direction that has beenmeasured in advance and interval L1 and, thereby, it becomes possible todetect the focus shift.

[0233] In addition, focus monitoring can also be carried out bymeasuring the electrical resistance of the pattern as follows.

[0234] In reference to FIGS. 16A, 16B and 17, a light blocking film 5 chaving an aperture pattern 5 d on the rear surface of a substrate 5 a inthe same manner as in the configuration of FIGS. 4A and 4B is formed ona photomask 5. In addition, a light blocking film 5 b having twoaperture patterns 5 b ₁ and 5 b ₂ that are arranged in an approximatelysymmetrical manner relative to aperture pattern 5 b on the rear surfaceof the substrate is formed on the top surface of substrate 5 a.

[0235] A double exposure with shift is carried out on photoresist 21 baccording to the principle shown in FIG. 1 by using the above describedphotomask 5. Concretely, wafer 21 is moved in the lateral direction (x-ydirection) after the first exposure is carried out as shown in FIG. 18Aand, then, a second exposure is carried out as shown in FIG. 18B. Due tothis double exposure with shift, a pattern, as shown in FIG. 18C, istranscribed onto the photoresist (negative type). Furthermore, theconductive film that is the lower layer of the resist pattern is etchedand patterned by using the resist pattern to which the above pattern istranscribed as a mask.

[0236] Thereby, the pattern of FIG. 18C is transcribed to the conductivefilm. The right edge of the fine line portion (portion of line width W)of the pattern in the center of FIG. 18C, which is transcribed onto theconductive film, is formed of aperture pattern 5 b ₂ due to the firstexposure while the left edge is formed of aperture pattern 5 b ₁ due tothe second exposure. Therefore, in the case that a positional shift iscaused by a shift in focus between the transcription position of thepattern due to the first exposure and the transcription position of thepattern due to the second exposure, the line width W of the fine lineportion fluctuates. Since the resistance value of the pattern of thisconductive layer changes because of the fluctuation of this line widthW, it becomes possible to measure the shift in focus by measuring thisresistance value.

[0237] The present inventors measured the mutual position shift of thetwo patterns for position measurement at the time when the height of thewafer is actually changed in the z direction by using the method of thepresent embodiment. The result thereof is shown in FIG. 19.

[0238] Here, this measurement is carried out by setting the numericalaperture NA in the exposure unit for exposing a photoresist to photomaskfor focus monitoring at 0.68 and by using a zonal illumination ofσ_(in)/σ_(out)=0.65/0.85.

[0239] In reference to FIG. 19, it can be seen that when the height ofthe wafer changes 1 μm in the z direction, the patterns move vis-à-viseach other by approximately 0.9 μm. Thereby, it can be seen that the zdetection sensitivity in the method of focus monitoring in the presentembodiment is 0.9, which is more sensitive than the z detectionsensitivity in conventional phase shift focusing monitor.

[0240] Next, the size of radius r of aperture pattern 5 d on the rearsurface of photomask 5, shown in FIG. 4, is described.

[0241] In reference to FIG. 20, when the aperture radius of the aperturepattern 5 d on the rear surface of the substrate is r, the thickness ofsubstrate 5 a is D, the numerical aperture is NA and the coherence,which is the interference indication of the exposure light, is σ, it ispreferable for the value of sin(tan⁻¹(r/D)) to be smaller than the INAvalue (=NA×σ/projection magnification), which is the amount ofillumination spread.

[0242] The above described sin(tan⁻¹(r/D)) indicates the scale of thespread of light components with which pattern 5 b ₁ (or 5 b ₂) forposition measurement is irradiated through aperture pattern 5 d on therear surface of the photomask. This is described in the following.

[0243] In reference to FIG. 20, first, the angle of light with whichpattern 5 b ₁ (or 5 b ₂ ) for position measurement is irradiated throughaperture pattern 5 d on the rear surface of the photomask is denoted asø, the scale of this spread of light components can be represented assin(ø/2). This ø becomes approximately equal to the angle _(χ) of thelight that proceeds in the direction of the optical axis throughaperture pattern 5 d on the rear surface of the photomask. This isbecause radius r of aperture pattern 5 d on the rear surface of thephotomask is sufficiently small in comparison with thickness D ofsubstrate 5 a (D>>r) and because distance d between point G on the topsurface that is directly opposite the center of aperture pattern 5 d onthe rear surface of the photomask and the center of pattern 5 b ₁ (or 5b ₂) for position measurement is sufficiently small in comparison withthickness D of substrate 5 a (D>>d).

[0244] Then, the scale of the spread of light components that enter inthe direction of the optical axis through aperture pattern 5 d on therear surface of the photomask can be represented as sin(_(χ)/2) wherethis _(χ)/2 is equal to tan⁻¹(r/D).

[0245] As described above, the scale sin(ø/2) of the spread of the lightcomponents with which pattern 5 b ₁ (or 5 b ₂ ) for position measurementthrough aperture pattern 5 d on the rear surface of the photomaskbecomes equal to sin(tan⁻¹(r/D)) as shown in the following equation.

sin(ø/2)=sin(_(χ)/2)=sin(tan⁻¹(r/D))

[0246] In addition, the INA value that is the amount of spread ofillumination is represented as sin(θ) in FIG. 20. Accordingly, whensin(tan⁻¹(r/D)) is smaller than the INA value it shows radius r ofaperture pattern 5 d on the rear surface of the photomask that can blocka portion of the light components (angle 2θ) that enter, in thedirection of the optical axis to pattern 5 b ₁ (or 5b₂) for positionmeasurement.

[0247] That is to say, in the case that the value of sin(tan⁻¹(r/D)) isthe INA value, or greater, a portion of the exposure light that enterspattern 5 b ₁ (or 5 b ₂) for position measurement cannot be blocked,depending on the arrangement position of aperture pattern 5 d on therear surface of the photomask, so that the incident directions of theexposure light that enters two patterns 5 b ₁ and 5 b ₂ for positionmeasurement cannot be made to differ.

[0248] In addition, it is preferable for the value of sin (tan⁻¹(r/D))to be greater than 0.1 times the INA value. This is because in the casethat the value of sin(tan⁻¹(r/D)) is 0.1 times the INA value, or less,the amount of exposure light becomes {fraction (1/100)}, or less, of thecase of the conventional transcription so that it becomes difficult totranscribe pattern 5 b ₁ (or 5 b ₂ ) for position measurement ontophotosensitive material and the throughput of the measurement for focusis lowered.

[0249] Here, though in the present embodiment the configuration whereinone aperture pattern 5 d on the rear surface of the photomask isprovided so as to be shared by two patterns 5 b ₁ and 5 b ₂ for positionmeasurement is described, the configuration of the photomask is notlimited to this but, rather, each of the two aperture patterns on therear surface of the photomask is provided so as to correspond to each ofthe two patterns for position measurement as described in the followingsecond embodiment.

[0250] Second Embodiment

[0251] In reference to FIGS. 21A, 21B and 22, this photomask 5 for focusmonitoring is provided with aperture patterns 5 d ₁ and 5 d ₂ on therear surface so as to correspond, respectively, to two patterns 5 b ₁and 5 b ₂ for position measurement. Thereby, as shown in FIG. 22, eachof the patterns, 5 b ₁ and 5 b ₂, for position measurement is irradiatedwith light components that have passed through the differing aperturepatterns 5 d ₁ and 5 d ₂ on the rear surface of the photomask.

[0252] Each of the diameters L0 of these two aperture patterns, 5 d ₁and 5 d ₂, on the rear surface of the photomask is formed so as tosatisfy the condition of L0/λ≧10 when the wavelength of the exposurelight that irradiates this photomask 5 is λ.

[0253] Two aperture patterns 5 d ₁ and 5 d ₂ on the rear surface of thephotomask are symmetrically arranged relative to point c2 on the rearsurface that is directly opposite to the central point c1 of thefictitious line (E0-E0) that connects two patterns 5 b ₁ and 5 b ₂ forposition measurement. In addition, two aperture patterns 5 d ₁ and 5 d ₂on the rear surface of the photomask are symmetrically arranged relativeto the fictitious line (E2-E2) that is drawn by projecting the line(E1-E1) that is perpendicular to, and divides into two equal segments,the fictitious line (E0-E0) connecting two patterns 5 b ₁ and 5 b ₂ forposition measurement onto the rear surface of the photomask.

[0254] In addition, the distance between each of the two aperturepatterns, 5 d ₁ and 5 d ₂, on the rear surface of the photomask and thefictitious line (E2-E2) is set so as to be longer than the distancebetween each of the two patterns, 5 b ₁ and 5 b ₂, for positionmeasurement and the fictitious line (E1-E1). Therefore, pattern 5 b ₁for position measurement is irradiated with light only from aperturepattern 5 d i on the rear surface of the photomask while pattern 5 b ₂for position measurement is irradiated with light only from aperturepattern 5 d ₂ on the rear surface of the photomask.

[0255] That is to say, as shown in FIG. 23, only aperture pattern 5 d ₁on the rear surface of the substrate is distributed within the rangefrom which pattern 5 b ₁ for position measurement can be irradiated withexposure light on the rear surface of substrate 5 a as is seen frompattern 5 b ₁ for position measurement. In addition, as shown in FIG.24, only aperture pattern 5 d ₂ on the rear surface of the substrate isdistributed within the range from which pattern 5 b ₂ for positionmeasurement can be irradiated with exposure light on the rear surface ofsubstrate 5 a as is seen from pattern 5 b ₂ for position measurement.

[0256] In the case that photomask 5 shown in FIGS. 21A, 21B and 22 isused, the respective two patterns 5 b ₁ and 5 b ₂ for measurement can beirradiated with components of the exposure light of differing diagonaldirections. Therefore, it is possible to carry out focus monitoring inthe same manner as the case wherein photomask 5, shown in FIGS. 4A and4B, is used.

[0257] In addition, the photomask for focusing may have a configurationwherein the respective two patterns 5 b ₁ and 5 b ₂ for measurement areilluminated from two aperture patterns 5 d ₁ and 5 d ₂ on the rearsurface of the photomask as described in the following third embodiment.

[0258] Third Embodiment

[0259] In reference to FIGS. 25A, 25B and 26, in this photomask 5, thedistance between each of the two aperture patterns, 5 d ₁ and 5 d ₂, onthe rear surface of the photomask and the fictitious line (E2-E2) is setto be shorter than the distance between each of the two patterns, 5 b ₁and 5 b ₂, for position measurement and the fictitious line (E1-E1).

[0260] Since two aperture patterns, 5 d ₁ and 5 d ₂, on the rear surfaceof the photomask are arranged in such a manner, pattern 5 b ₁ forposition measurement is irradiated with the exposure light that haspassed through both aperture patterns 5 d ₁ and 5 d ₂ on the rearsurface of the photomask as shown in FIG. 26 while pattern 5 b ₂ forposition measurement is also irradiated with the exposure light that haspassed to aperture pattern, 5 d ₁ and 5 d ₂, on the rear surface of thephotomask.

[0261] That is to say, as shown in FIG. 27, aperture patterns 5 d i and5 d ₂ on the rear surface of the photomask are distributed within therange wherein pattern 5 b ₁ for position measurement on the rear surfaceof substrate 5 a can be irradiated with the exposure light as can beseen from pattern 5 b ₁ for position measurement. In addition, as shownin FIG. 28, aperture patterns 5 d ₁ and 5 d ₂ on the rear surface of thephotomask are distributed within the range wherein pattern 5 b ₂ forposition measurement on the rear surface of substrate 5 a can beirradiated with the exposure light as can be seen from pattern 5 b ₂ forposition measurement.

[0262] The parts of the configuration other than the above areapproximately the same as the configuration shown in FIGS. 21A, 2 1B and22 and, therefore, the same symbols are attached to the same members, ofwhich the descriptions are omitted.

[0263] In this photomask 5 shown in FIGS. 25A, 25B and 26, each of thetwo patterns, 5 b ₁ and 5 b ₂, for position measurement can beirradiated with the components of exposure light of differing diagonaldirections and, therefore, it is possible to carry out focus monitoringin the same manner as in the case wherein photomask 5 shown in FIGS. 4Aand 4B is used.

[0264] In addition, though a circular aperture pattern is described as apattern on the rear surface of the photomask in reference to FIGS. 4A,4B and 5, a circular pattern that allows a light blocking film to remainas described in the following fourth embodiment may be used.

[0265] Fourth Embodiment

[0266] In reference to FIGS. 29A, 29B and 30, a light blocking film 5 con which a circular pattern 5 d that allows a light blocking film toremain is formed on the rear surface of a substrate 5 a. In addition,two patterns 5 b ₁ and 5 b ₂ for position measurement are arrangedwithin the top surface region of substrate 5 a, which opposes the regionwherein the above pattern 5 d is formed allowing a light blocking filmto remain.

[0267] The diameter 2 r of this pattern 5 d that allows a light blockingfilm to remain is formed so as to satisfy the condition of 2 r/λ≧10 whenthe wavelength of the exposure light with which this photomask 5 isirradiated is λ.

[0268] Here, the parts of the configuration other than the above areapproximately the same as the configuration of the above describedphotomask of FIGS. 4A, 4B and 5 and, therefore, the same symbols areattached to the same members, of which the descriptions are omitted.

[0269] In this photomask 5 both of the two patterns, 5 b ₁ and 5 b ₂,for position measurement are arranged within the top surface region ofsubstrate 5 a that is directly opposite to the region wherein a circularpattern 5 d that allows a light blocking film to remain is formed.Therefore, as shown in FIG. 30, each of these two patterns, 5 b ₁ and 5b ₂, for position measurement is irradiated with components of theexposure light of differing diagonal directions

[0270] That is to say, as shown in FIG. 31, pattern 5 d that allows alight blocking film to remain is positioned only in a portion within theregion that allows the irradiation of pattern 5 b ₁ for positionmeasurement with the exposure light on the rear surface of substrate 5 aas is seen from pattern 5 b ₁ for position measurement. In addition, asshown in FIG. 32, pattern 5 d that allows a light blocking film toremain is positioned only in a portion within the region that allows theirradiation of pattern 5 b ₂ for position measurement with the exposurelight on the rear surface of substrate 5 a as is seen from pattern 5 b ₂for position measurement.

[0271] With the configuration of this photomask 5, it becomes possibleto irradiate both of the two patterns, 5 b ₁ and 5 b ₂, for positionmeasurement with the exposure light of differing diagonal directions asdescribed above and, therefore, it becomes possible to carry out focusmonitoring in the same manner as with photomask 5 shown in FIGS. 4A, 4Band 5.

[0272] Here, it is preferable for the value of sin(tan⁻¹(r/D)) to besmaller than the INA value when the radius of pattern 5 d that allows alight blocking film to remain is denoted as r. This is because, in thecase that the value of sin(tan⁻¹(r/D)) is of the INA value, or greater,some portion of pattern 5 d that allows a light blocking film to remainarranged in a specific position cannot be irradiated with a portion ofthe exposure light that enters patterns 5 b ₁ and 5 b ₂ for positionmeasurement so that the incident directions of the exposure light thatenters two patterns, 5 b ₁ and 5 b ₂, for position measurement cannot bemade to differ.

[0273] In addition, it is preferable for the value of sin(tan⁻¹(r/D)) tobe greater than 0.5 times the INA value. In the case that the value ofsin(tan⁻¹(r/D)) is 0.5 times the INA value, or smaller, the lightblocking portion becomes too small and, therefore, it becomes difficultto secure the non-telecentric characteristics of the exposure light sothat the detection sensitivity of the patterns in focus monitoring islowered.

[0274] In addition, though a circular aperture pattern is described as apattern on the rear surface of the photomask in FIGS. 4A, 4B and 5, arectangular light blocking pattern may be used as described in thefollowing fifth embodiment.

[0275] Fifth Embodiment

[0276] In reference to FIGS. 33A, 33B and 34, a light blocking film 5 c,on which a rectangular pattern 5 d that allows a light blocking film toremain is formed, is formed on the rear surface of a substrate 5 a. Inaddition, two patterns, 5 b ₁ and 5 b ₂, for position measurement arearranged within the top surface region of substrate 5 a that is directlyopposite to the region wherein the above pattern 5 d that allows a lightblocking film to remain is formed.

[0277] The length a of one side of this pattern 5 d that allows a lightblocking film to remain is formed so as to satisfy the condition ofa/λ≧10 when the wavelength of the exposure light, with which thisphotomask 5 is irradiated, is λ.

[0278] Here, the parts of the configuration other than the above areapproximately the same as the above described configuration of photomask5 of FIGS. 4A, 4B and 5 and, therefore, the same symbols are attached tothe same members, of which the descriptions are omitted.

[0279] Both of the two patterns, 5 b ₁ and 5 b ₂, for positionmeasurement are arranged within the top surface region of substrate 5 athat is directly opposite to the region wherein a rectangular pattern 5d that allows a light blocking film to remain is formed in thisphotomask 5. Therefore, each of these two patterns, 5 b ₁ and 5 b ₂, forposition measurement is irradiated with components of the exposure lightof differing diagonal directions as shown in FIG. 34.

[0280] That is to say, pattern 5 d that allows a light blocking film toremain is positioned in only a portion within the region on the rearsurface of substrate 5 a wherein pattern 5 b ₁ for position measurementcan be irradiated with the exposure light as is seen from pattern 5 b ₁for position measurement. In addition, pattern 5 d that allows a lightblocking film to remain is positioned in only a portion within theregion on the rear surface of substrate 5 a wherein pattern 5 b ₂ forposition measurement can be irradiated with the exposure light as isseen from pattern 5 b ₂ for position measurement.

[0281] With the configuration of this photomask 5, it becomes possibleto irradiate the both of the above described two patterns, 5 b ₁ and 5 b₂, for position measurement with the exposure light of differingdiagonal directions and, therefore, it becomes possible to carry outfocus monitoring in the same manner as with photomask 5 shown in FIGS.4A, 4B and 5.

[0282] Here, it is preferable for pattern 5 d that allows a lightblocking film to remain to have an aperture wherein a light blockingfilm is not formed within the range of R wherein the distance Rvis-à-vis a side that forms the external form of pattern 5 d that allowsa light blocking film to remain satisfies the relationship ofsin(tan⁻¹(R/D))≧INA value.

[0283] In addition, when the length of the shorter sides of pattern 5 dthat allows a light blocking film to remain is denoted as a, it ispreferable for the value of sin(tan⁻¹(a/D)) to be greater than 0.5 timesin the INA value. In the case that the value of sin(tan⁻¹(a/D)) is 0.5times the INA value, or smaller, the light blocking portion becomes toosmall and, therefore, it becomes difficult to secure the non-telecentriccharacteristics of the exposure light so that the detection sensitivityof the patterns in focus monitoring is lowered.

[0284] In addition, though a circular aperture pattern is described as apattern on the rear surface of the photomask in reference to FIGS. 4A,4B and 5, a rectangular aperture pattern may be used as described in thefollowing sixth embodiment.

[0285] Sixth Embodiment

[0286] In reference to FIGS. 35A and 35B, a light blocking film 5 c thatforms a rectangular aperture pattern 5 d is formed on the rear surfaceof a substrate 5 a. Rectangular aperture pattern 5 d is formed in arectangular form (for example, square form) of which the center is pointc2 on the rear surface that is directly opposite to the center point clof the fictitious line connecting the above two patterns 5 b ₁ and 5 b ₂for position measurement. In addition, the cross section correspondingto line V-V in FIGS. 35A and 35B is the same as of the configuration inFIG. 5.

[0287] The length a of one side of the above rectangular aperturepattern 5 d is formed so as to satisfy the condition of a/λ≧10 whereinthe wavelength of the exposure light with which this photomask 5 isirradiated is λ.

[0288] Here, the other parts of the configuration are approximately thesame as the configuration of the above described photomask in FIGS. 4A,4B and 5 and, therefore, the same symbols are attached to the samemembers, of which the descriptions are omitted.

[0289] In this photomask 5, patterns 5 b ₁ and 5 b ₂ for positionmeasurement are arranged on both sides, respectively, of the opticalaxis (line A-A) that passes through rectangular aperture pattern 5 d.Therefore, the incident direction of the exposure light that enterspattern 5 b ₁ for position measurement after passing through rectangularaperture pattern 5 d and the incident direction of the exposure lightthat enters pattern 5 b ₂ for position measurement differ from eachother. In particular, in the case that patterns 5 b ₁ and 5 b ₂ forposition measurement are symmetrically arranged relative to the opticalaxis A-A that passes through rectangular aperture pattern. 5 d, theincident direction of the exposure light that enters pattern 5 b ₁ forposition measurement and the incident direction of the exposure lightthat enters pattern 5 b ₂ for position measurement are mutuallysymmetric relative to the direction of the optical axis A-A.

[0290] In such a manner each of the two patterns, 5 b ₁ and 5 b ₂, forposition measurement is irradiated with a differing component of thediagonal direction of the exposure light.

[0291] It becomes possible to irradiate both of the two patterns 5 b ₁and 5 b ₂ for position measurement with exposure light from differingdiagonal directions as described above in the configuration of the abovephotomask 5 and, therefore, it becomes possible to carry out focusmonitoring in the same manner as in photomask 5 shown in FIGS. 4A, 4Band 5.

[0292] Here, it is preferable for the light blocking portion to be alight blocking film that is formed within a range wherein distance Rvis-à-vis a side that forms the external form of rectangular aperturepattern 5 d satisfies the relationship of sin(tan⁻¹(R/D))≧INA value.Thereby, illumination light from rectangular aperture pattern 5 d alonecan be allowed to enter each of the two patterns 5 b ₁ and 5 b ₂ forposition measurement so that the non-telecentric characteristics of theexposure light can be firmly secured.

[0293] In addition, it is preferable for the value of sin(tan⁻¹(a/D)) tobe greater than 0.2 times the INA value wherein the length of theshorter side of rectangular aperture pattern 5 d is a. In the case thatthe value of sin(tan⁻¹(a/D)) is 0.2 times the INA value, or smaller, theamount of the exposure light becomes approximately {fraction (1/100)} ofthe case of a conventional transcription and it becomes difficult totranscribe a pattern for position measurement to a photoresist so thatthe throughput of the measurement of the focus is lowered.

[0294] Seventh Embodiment

[0295] In reference to FIGS. 36A, 36B and 37, a photomask 5 for focusmonitoring of the present embodiment is different from the configurationshown in FIGS. 35A and 35B in the point that each of patterns 5 b ₁ and5 b ₂ for position measurement is positioned on the top surface side ofsubstrate 5 a that is directly opposite to the middle point (c3, c4) ofeach of the two sides facing each other of square aperture pattern 5 d.

[0296] Here, the other parts of the configuration are approximately thesame as the configuration of the above described FIGS. 35A and 35B and,therefore, the same symbols are attached to the same members, of whichthe descriptions are omitted.

[0297] In the present embodiment each other of the two patterns, 5 b ₁and 5 b ₂, for position measurement is positioned on the top surfaceside of substrate 5 a that is directly opposite to the center point (c3,c4) of a side square aperture pattern 5 d. Therefore, approximately halfof the complementary exposure light with which each of two patterns, 5 b₁ and 5 b ₂, for position measurement is irradiated as shown in FIG. 37is blocked by a light blocking film 5 c, while each of the patterns, 5 b₁ and 5 b ₂, for position measurement is irradiated with only theremaining half.

[0298] In addition, square aperture pattern 5 d is positioned as shownin FIG. 38 on the rear surface side of substrate 5 a as seen frompattern 5 b ₁ for position measurement in this photomask 5. In addition,square aperture pattern 5 d is positioned as shown in FIG. 39 on therear surface side of substrate 5 a as seen from pattern 5 b ₂ forposition measurement. Therefore, in the case that a zonal illuminationdiaphram 14 shown in FIG. 6 is used as an diaphram 14 of FIG. 3, thelight that illuminates pattern 5 b ₁ for position measurement is solelythe illumination light of the right half of the zonal illumination asshown in FIG. 40 and the exposure light with which pattern 5 b ₂ forposition measurement is irradiated is solely the illumination light ofthe left half of the zonal illumination as shown in FIG. 41.

[0299] Thereby, the respective patterns, 5 b ₁ and 5 b ₂, for positionmeasurement are irradiated with the light from diagonal directionsdiffering from each other and it becomes possible to carry out focusmonitoring in the same manner as in the first embodiment.

[0300] In the case that photomask 5 shown in FIGS. 36A, 36B and 37 isexposed, it is preferable for the light blocking film to be formed in alight blocking portion within the range R wherein the distance vis-à-visthe side that forms the external form of aperture pattern 5 d on therear surface of the photomask satisfies sin(tan⁻¹(R/D))≧INA value whenthe thickness of substrate 5 a is D, the numerical aperture is NA, andthe coherence that is the interference index of the exposure light is σ.Thereby, only the illumination light from square aperture pattern 5 d isallowed to enter into each of the two patterns, 5 b ₁ and 5 b ₂, forposition measurement so that the non-telecentric characteristics of theexposure light can be firmly secured.

[0301] In addition, it is preferable for the value of sin(tan⁻¹(a/D)) tobe greater than two times the INA value when the length of one side ofsquare aperture pattern 5 d is a. Thereby, it is possible for theillumination light components that enter patterns 5 b ₁ and 5 b ₂ forposition measurement on the top surface of the photomask to be alwaysapproximately half of the entirety of the illumination with respect toan arbitrary illumination form.

[0302] In addition, it is preferable for the value of sin(tan⁻¹(a/D)) tobe smaller than three times the INA value. In the case that the value ofsin(tan⁻¹(a/D)) is three times the INA value, or greater, squareaperture pattern 5 d on the rear surface of the photomask becomes toolarge so that it becomes difficult to arrange a large number of unitmask structures for focus monitoring on the mask.

[0303] In the case that one pattern is illuminated by using each of thehalf illuminations of the right and left sides such as, for example, aphotomask of the present embodiment, the change in the image intensitydue to change in the focus position becomes as shown in FIG. 42A, 42Band 42C. In reference to FIGS. 42A, 42B and 42C, the intensity of theimage resulting from the total illumination of the half illumination onthe right side and the half illumination on the left side variesdepending on the position of the wafer in the z direction. That is tosay, when the wafer is in the defocused position, the image resultingfrom the half illumination on the right side and the image resultingfrom the half illumination on the left side cause mutual positionalshift so that the intensity of the image resulting from the entirety ofthe illumination becomes small. Contrarily, in the case that the waferis in the optimally focused position, the image resulting from the halfillumination on the right side and the image resulting from the halfillumination on the left side do not cause a positional shift so thatthey approximately agree with each other and, therefore, the imageresulting from the entirety of the illumination attains optimalsharpness and the image intensity becomes of the maximum.

[0304] In the case that the exposure is carried out by using such halfilluminations on the right and left sides, the optimally focusedposition (height position in the z direction wherein the positions ofthe images resulting from the respective half illuminations agree witheach other) agrees with the optimally focused position (height positionin the z direction wherein the intensity of the dimension/image, or thelike, becomes of the extreme value) of the actual transcriptionresulting from the total illumination. This is described in detail inthe following.

[0305] The position of the optimal focus for the image formation becomesdisplaced from the optimal focus position in the ideal lens systembecause of the lens aberration. The lens aberration is represented as aphase error in the diaphram plane. This phase error can be considered bybeing divided into even aberration wherein the same values are gained intwo points that are symmetrical relative to the center and oddaberration wherein the signs of the values are made opposite to eachother. As for the even aberration and odd aberration, a change in focusis caused by the even aberration. On the other hand, the odd aberrationcauses a shift in the image in the lateral direction.

[0306] In reference to FIG. 43, each component illumination of the halfillumination of the right side and the half illumination of the leftside are mutually positioned according to rotational symmetry relativeto the diaphram center in the diaphram plane. As shown in the figure,the rays are in the normal direction relative to the equiphase wavesurface. In addition, the positions of the respective half illuminationon the right side and left side are symmetrical and the diffracted lightis generated symmetrically around the 0 order diffracted light(non-refracted illumination light) by means of the non-phase shift mask.Therefore, in the image formations resulting from the respectivecomponent illuminations, the optimal focus positions shift by the sameamount in the z direction as shown by the dotted line. That is to say,in these shifted optimal focus positions, the mutual positional shift ofthe patterns formed by the half illumination on the left side and thehalf illumination on the right side becomes 0 and becomes the same asthe ideal condition (no aberration and optimal focus).

[0307] On the other hand, the total illumination is the sum of the halfillumination on the left side and the half illumination on the rightside. Therefore, the case of image formation resulting from the totalillumination is considered in the same manner as the case wherein apattern is irradiated by being divided into the half illumination on theleft side and the half illumination on the right side. Accordingly, inthe case that only the even aberration exits, the optimal focus,position resulting from the total illumination shifts as shown by thedotted line of FIG. 43. Accordingly, even when there is an evenaberration, the optimal focus positions wherein images are separatelyformed by the half illumination on the left side and the halfillumination on the right side and the optimal focus position whereinthe images formed by the total illumination become the same.Accordingly, it is understood that the even aberration does not cause adifference between the optimal focus according to the present method offocus monitoring and the optimal focus in the actual transcriptionresulting from the total illumination.

[0308] Next, the effects of the odd aberration are considered.

[0309] In the actual image formation it is necessary to consider thelateral shift of a pattern due to the odd aberration. As shown in FIG.44 when respective component illuminations that form the halfillumination on the left side and the half illumination on the rightside is considered, the images resulting from the two componentilluminations laterally shift by the same amount and in the samedirection due to the symmetry of the diffracted light and theaberration. That is to say, the pattern formed by the half illuminationon the left side and the pattern formed by the half illumination on theright side do not relatively shift in the lateral direction due to theodd aberration.

[0310] It is understood from the above that the odd aberration does notcause a disturbance to the measurement of the optimal focus positionaccording to the method of focus monitoring of the present embodiment.

[0311] As described above, the optimal focus position is determinedsolely by the even aberration, even when the odd aberration exists, andthe optimal focus position of the image resulting from the totalillumination and the substrate position in the z direction, whereinrelative positional shift between the respective patterns resulting fromthe half illumination on the left side and the half illumination on theright side becomes 0 as in the present embodiment, agree with eachother.

[0312] Next, the working effects of focus monitoring in the method offocus monitoring according to the present embodiment are described byusing a conceptual diagram.

[0313]FIG. 45 is a diagram showing the behavior of the transcriptionpositions of the mask pattern in the method of focus monitoringaccording to the seventh embodiment of the present invention when onlythe even aberration exists, wherein the lateral shift of the imagepatterns formed by the half illumination on the left side and the halfillumination on the right side is shown as a function of the height(distance vis-à-vis the optical system) of the surface of thetranscribed substrate.

[0314] In reference to FIG. 45, a solid line, a dotted line and a brokenline that extend from the upper left to the lower right show thepositions of the images resulting from the half illumination on the leftside while a solid line, a dotted line and a broken line a that extendfrom the upper right to the lower left are lines showing the positionsof the images resulting from the half illumination on the right side. Inthe case that the even aberration exists, the height wherein the lateralshift of the pattern becomes 0 changes in the same manner for eitherpattern resulting from the half illumination on the left side or thepattern resulting from the half illumination on the right side.

[0315] In reference to FIG. 46, it is understood that the position ofthe optimal focus can be found from the substrate height wherein therelative shift becomes 0.

[0316] In reference to FIG. 47, either the pattern resulting from thehalf illumination on the right side or the pattern resulting from thehalf illumination on the left side shifts by the same amount and in thesame direction due to the odd aberration. Therefore, in the case thatthe odd aberration exists, a condition is gained wherein the conditionof the optimal focus has shifted by a specific amount in the upward ordownward direction in the figure in comparison with the case wherein theeven aberration alone exists as shown in FIG. 45.

[0317] In addition, the mutual positional shift of the patternsresulting from the respective half illuminations on the right side andthe left side that is the experimental measurement amount becomes thesame as in the case that (FIG. 46) wherein there is no odd aberrationsince the pattern resulting from the half illumination on the right sideand the pattern resulting from the half illumination on the left sidelaterally shift in the same manner according to the amount ofaberration, as shown in FIG. 48.

[0318] The behavior of the actual transcription pattern that correspondsto the behavior of the above described focus monitoring is shown inFIGS. 49A and 49B by citing an example of the CD (criticaldimension)—focus characteristics.

[0319] In reference to FIG. 49A, the total illumination is the same asthe simultaneous illuminations of pattern 5 b ₃ of the above describedhalf illumination on the right side and the half illumination on theleft side. Then, in this case, as shown in FIG. 49B, it is understoodthat the position of the optimal focus, in the case that the evenaberration exists, fluctuates in the direction of the optical axis (zdirection) in comparison with the case wherein the even aberration doesnot exist. Though the optimal focus position can be found by using theCD-focus characteristics in the above manner, it is necessary, forexample, to find the peak position of such moderate characteristics sothat a large amount of labor is required and the precision is lowered.

[0320] As described above, even in the case that the exposure is carriedout by using the half illuminations on the left and right sides, theposition of the optimal focus agrees with the optimal focus position ofthe actual transcription resulting from the total illumination. Thereby,the optimal focus position of the actual transcription resulting fromthe total illumination can be found by using the half illuminations onthe left and right sides. That is to say, though the position of theoptimal focus in the actual transcription must be found from thecontrast of the images in the case that it is found from the actualtranscription pattern, it can be found from the relative positionalshift of the two images of the patterns by using the method of focusmonitoring of the present embodiment and, therefore, it becomes possibleto find in a simpler manner and with a higher precision than in the casethat it is found from the actual transcription pattern.

[0321] Though in the present embodiment, the case is described whereinaperture pattern 5 d in a rectangular form (square form) on the rearsurface of the photomask is used, the form of the rear surface patternis not limited to this but, rather, a pattern such as is described inthe following eighth embodiment may be used.

[0322] Eighth Embodiment

[0323] In reference to FIGS. 50A, 50B and 51, a photomask 5 of thepresent embodiment is different from photomask 5 shown in FIGS. 36A, 36Band 37 in the point that a pattern 5 d, which allows a light blockingfilm to remain in a rectangular form (for example, square form), isformed of a light blocking film 5 c as a rear surface pattern.

[0324] In the case that this pattern 5 d that allows a light blockingfilm to remain is in a square form, two patterns 5 b ₁ and 5 b ₂ forposition measurement are respectively arranged in positions on the topsurface side that are directly opposite to the center points (c3, c4) ofthe two sides that facing each other in the above square.

[0325] Here, the other parts of the configuration are approximately thesame as the above described configuration in FIGS. 36A, 36B and 37 and,therefore, the same symbols are attached to the same members, of whichthe descriptions are omitted.

[0326] It is preferable for the value of sin(tan⁻¹(a/D)) to be largerthan two times the INA value wherein the length of one side of thesquare of the above pattern that allows a light blocking film to remainon the rear surface is a. Thereby, the instant angle component of theillumination on the side where a light blocking pattern exists can becompletely blocked so as not to reach patterns 5 b ₁ and 5 b ₂ forposition measurement.

[0327] In addition, it is preferable for the value of sin(tan⁻¹(a/D)) tobe smaller than three times the INA value. In the case that the value ofsin(tan⁻¹(a/D)) is three times the INA value, or greater, the lightblocking pattern becomes large when this is not necessary, thedimensions of the unit mask structures Q for focus monitoring becomelarge so that a great number of the above described structures Q cannotbe arranged on the mask and, therefore, it is desirable to make theabove value three times, or smaller.

[0328] Each of patterns 5 b ₁ and 5 b ₂ for position measurement can beirradiated with the half illumination as shown in FIG. 51 with thephotomask shown in FIGS. 50A, 50B and 51. Therefore, it is possible tocarry out focus monitoring in the same manner as with photomask 5 shownin FIGS. 36A, 36B and 37.

[0329] Though in the above described first to eighth embodiments,pattern 5 b ₁ for position measurement is a square aperture pattern usedas an inner box pattern while pattern 5 b ₂ for position measurement isan aperture pattern in a square frame form used as an outer box pattern,two patterns, 5 b ₁ and 5 b ₂, for position measurement are not limitedto these forms.

[0330] Two patterns, 5 b ₁ and 5 b ₂, for position measurement may beline patterns that have a light blocking film remaining in a squareframe form as shown in FIG. 52, may be space patterns formed asapertures in square frame forms as shown in FIG. 53 or may be patternswherein a plurality of hole patterns are arranged in a square form asshown in FIG. 54.

[0331] In addition, in the first to eighth embodiments, it is preferablefor the size S of the pattern in the box edge of, at least, either theinner box pattern or the outer box pattern to satisfy S=k₁×λ/NA(0.3<k₁<0.6) when the constant, which depends on the resist process andthe image formation conditions, is k₁, the wavelength of the exposurelight is λ and the numerical aperture is NA. Thereby, the measurement offocus corresponding to the actual device becomes possible.

[0332] In addition, a plurality of focus monitoring unit mask structuresQ provided with two patterns, 5 b ₁ and 5 b ₂, for position measurementshown in FIGS. 4A and 4B and aperture pattern 5 d on the rear surface ofthe photomask may be arranged in photomask 5 as shown in FIG. 55. Inthis case, it is preferable for the pitch P between focus monitoringunit mask structures Q to be no less than 8 mm and no more than 20 mm.This is because such an arrangement makes it possible to sequentiallymeasure the focus distribution within the exposure field.

[0333] In addition, in the first to eighth embodiments, it is preferablefor distance M between two patterns, 5 b ₁ and 5 b ₂, for positionmeasurement to be greater than 0.5 times, and smaller than four times,the product (=INA value×D) of the INA value (=NA ×σ/projectionmagnification) that is the amount of spread of illumination and D whenthe thickness of the substrate is D, the numerical aperture is NA andthe coherence that is the interference indication of the exposure lightis σ. Thereby, the patterns for position measurement can beappropriately irradiated. In the case that distance M between the twopatterns for position measurement is 0.5 times the INA value×D, or less,the difference of the incident angles of the illumination light withwhich the two patterns for position measurement are irradiated cannot besufficiently secured. In addition, increasing the distance M between thetwo patterns for position measurement to four times the INA value×D, orgreater, is substantially meaningless and a large number of structurescannot be arranged on the mask since unit mask structures for focusmonitoring become large.

[0334] In addition, in FIG. 14, the maximum value Lmax of the distancebetween arbitrary mask structures N for correcting the amount of waferposition shift is greater than ½ of the dimension of the longitudinaldirection of the exposure region for one shot when the pattern istranscribed to the photosensitive material. Thereby, in the case thatthe exposure region for one shot is exposed while being shifted in therotational direction, the detection sensitivity of the shift amount inthe rotational direction can be increased.

[0335] In addition, in the first to eighth embodiments, it is preferablefor the step of measuring the distance between the respective imagepatterns of two patterns, 5 b ₁ and 5 b ₂, for position measurement thathave been transcribed to the resist pattern to be carried out by usingan overlap inspection unit for inspecting the positional shift of theoverlapping by processing the images of the two image patterns that havebeen read in. Thereby, the positional shift can be measured with a highprecision.

[0336] In addition, in the first to eighth embodiments, it is preferablefor the step of measuring the distance between the mutual image patternsof two patterns, 5 b ₁ and 5 b ₂, for position measurement that havebeen transcribed to the resist pattern to be carried out by observingthe positions of the two image patterns by means of a scanning-typeelectron microscope. Thereby, the positional shift can be easilymeasured.

[0337] In addition, in the first to eighth embodiments, it is preferablefor two patterns, 5 b ₁ and 5 b ₂, for position measurement to beconstructed so that the image pattern of, at least, either two patterns,5 b ₁ and 5 b ₂, for position measurement becomes readable by means of apattern position measurement mechanism that is integrally attached tothe exposure unit. Thereby, measurement becomes possible by means of aunit of a simple configuration.

[0338] In addition, though a zonal illumination diaphram is described asillumination diaphram 14 in the above description, illumination diaphram14 is not limited to this but, rather, it may be a modified illuminationsuch as a quadruple polar illumination diaphram or an ordinaryillumination other than the zonal illumination.

[0339] An actual transcription can be carried out with a precise focusin reference to the results gained in the first to sixth embodiments.

[0340] Here, the photoresist on the wafer surface is developed afterbeing exposed with the optimal focus gained according to the method offocus monitoring of the above described seventh and eighth embodiments,thereby, the photoresist is patterned so that processes, such as etchingor ion injection, can be carried out on the film that is the lower layerof the resist pattern by using the resist pattern so as to manufacture adesired semiconductor device with a high precision.

[0341] In addition, by utilizing a method of focus monitoring accordingto the present invention, other devices (units) such as a thin filmmagnetic head or a liquid crystal display element in addition to asemiconductor device can be formed with a high precision.

[0342] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A photomask for focus monitoring used for focus monitoring that measures the position of an exposed surface in an optical system in order to adjust the focus of an optical image on the exposed surface at the time of pattern exposure, comprising: a substrate that allows the exposure light to pass through and unit mask structures for focus monitoring, wherein said unit mask structure for focus monitoring has: two patterns for position measurement for measuring the mutual positional relationship formed on the surface of said substrate: and a light blocking film that is formed on the rear surface of said substrate and has a rear surface pattern for substantially differentiating the incident directions of the exposure light that enters said two patterns for position measurement, and wherein L/λ is 10, or greater, wherein the dimension of said rear surface pattern is L and the wavelength of the exposure light is λ.
 2. The photomask for focus monitoring according to claim 1, characterized in that said rear surface pattern formed on said light blocking film is formed so as to block a portion of the exposure light that would enter at least either of said two patterns for position measurement in the case that said light blocking film were not formed and so as to allow only the remaining portion of the exposure light to enter.
 3. The photomask for focus monitoring according to claim 1, characterized in that said rear surface pattern is a group of patterns that are shared by said two patterns for position measurement.
 4. The photomask for focus monitoring according to claim 3, characterized in that said rear surface pattern is a rectangular aperture pattern.
 5. The photomask for focus monitoring according to claim 4, characterized in that said rectangular aperture pattern is a square aperture pattern.
 6. The photomask for focus monitoring according to claim 5, wherein said two patterns for position measurement are respectively positioned at those positions of said surface which are directly opposite to mid point of each of two opposing sides of said square aperture pattern.
 7. The photomask for focus monitoring according to claim 6, wherein the value sin (tan⁻¹(a/D)) is larger than 2.0 times the INA value (=NA×σ/projection magnification) representing spread of illumination, where a represents length of one side of said square aperture pattern of said rear surface pattern, D represents thickness of said substrate, NA represents numerical aperture and σ represents coherence as an index of interference of the exposure light.
 8. The photomask for focus monitoring according to claim 3, characterized in that said rear surface pattern is a rectangular pattern that allows a light blocking film to remain.
 9. The photomask for focus monitoring according to claim 8, characterized in that said rectangular pattern that allows a light blocking film to remain is a square pattern that allows a light blocking film to remain.
 10. The photomask for focus monitoring according to claim 9, wherein said two patterns for position measurement are respectively positioned at those positions of said surface which are directly opposite to mid point of each of two opposing sides of said square pattern that allows a light blocking film to remain.
 11. The photomask for focus monitoring according to claim 10, wherein the value sin (tan⁻¹(a/D)) is larger than 2.0 times the INA value (=NA×σ/projection magnification) representing spread of illumination, where a represents length of one side of said square pattern that allows a light blocking film to remain of said rear surface pattern, D represents thickness of said substrate, NA represents numerical aperture and σ represents coherence as an index of interference of the exposure light.
 12. The photomask for focus monitoring according to claim 1, characterized in that one of said two patterns for position measurement is an inner box pattern of box-in-box type mark and the other one of said two patterns for position measurement is an outer box pattern.
 13. The photomask for focus monitoring according to claim 1, characterized by further comprising a mask structure for correcting wafer position shift amount that has two additional patterns for position measurement in order to measure the mutual positional relationships formed on the surface of said substrate and patterns formed on the light blocking film formed on the rear surface of said substrate in order to substantially equalize the incident directions of the exposure light that enters said two additional patterns for position measurement.
 14. The photomask for focus monitoring, according to claim 1, characterized in that a plurality of said unit mask structures for focus monitoring are formed on said substrate and the pitch of the two adjoining unit mask structures for focus monitoring is no less than 8 mm and no more than 20 mm.
 15. A method of focus monitoring used for focus monitoring that measures the position of an exposed surface in an optical system in order to adjust the focus of an optical image on the exposed surface at the time of pattern exposure, wherein focus monitoring is carried out by utilizing the characteristics wherein an image of the pattern of a photomask for focus monitoring formed on said photosensitive material surface by irradiating said photomask with the exposure light moves in the direction perpendicular to the optical axis when said photosensitive material surface is moved in the direction of said optical axis, wherein said photomask for focus monitoring comprises a substrate that allows the exposure light to pass through and unit mask structures for focus monitoring, wherein said unit mask structure for focus monitoring has: two patterns for position measurement for measuring the mutual positional relationship formed on the surface of said substrate; and a light blocking film that is formed on the rear surface of said substrate and that has a rear surface pattern for substantially differentiating the incident directions, relative to said substrate, of the exposure light that enters said two patterns for position measurement, and wherein L/λ is 10, or greater, wherein the dimension of said rear surface pattern is L and the wavelength of the exposure light is λ.
 16. The method of focus monitoring according to claim 15, characterized by comprising the steps of: applying a photoresist as said photosensitive material to the substrate; exposing said applied photoresist to images of said two patterns for position measurement of said photomask for focus monitoring; patterning said exposed photoresist through development so as to form a resist pattern; and focus monitoring based on the mutual distance between image patterns of said respective two patterns for position measurement that have been transcribed to the resist pattern.
 17. The method of focus monitoring according to claim 16, characterized in that said step of exposing said applied photoresist to the images of said two patterns for position measurement of said photomask for focus monitoring comprises: the first exposure step of exposing said photoresist to the images of said two patterns for position measurement of said photomask for focus monitoring; the step of moving said substrate to which said photoresist is applied in the direction perpendicular to said direction of the optical axis; and the second exposure step of exposing said photoresist to the images of said two patterns for position measurement of said photomask for focus monitoring, wherein either of one of the images of said two patterns for position measurement to which said photoresist is exposed in said second exposure step overlaps the other one of the images of said two patterns for position measurement to said photoresist is exposed in said first exposure step.
 18. A unit for focus monitoring used for focus monitoring that measures the position of an exposed surface in an optical system in order to adjust the focus of an optical image on the exposed surface at the time of pattern exposure, comprising: a photomask for focus monitoring on which a pattern is formed; an illumination optical system for irradiating said photomask for focus monitoring with exposure light; and a projection optical system for projecting an image of the pattern of said photomask for focus monitoring on a photosensitive material, wherein said photomask for focus monitoring comprises a substrate that allows the exposure light to pass through and unit mask structures for focus monitoring, wherein said unit mask structure for focus monitoring has: two patterns for position measurement for measuring the mutual positional relationship formed on the surface of said substrate; and a light blocking film that is formed on the rear surface of said substrate and that has a rear surface pattern for substantially differentiating the incident directions, relative to said substrate, of the exposure light that enters said two patterns for position measurement, and wherein L/λ is 10, or greater, where the dimension of said rear surface pattern is L and the wavelength of the exposure light is λ.
 19. A manufacturing method for a unit, characterized in that the method of focus monitoring according to claim 15 is used. 